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ELEMENTS 


OF 


GEOLOGY 


BY 


CHARLES  LYELL,  F.R.S. 

VICE-PRESIDENT  OF  THE  GEOLOGICAL  SOCIETY  OF  LONDON,  ETC.  ; 
AUTHOR   OF   "PRINCIPLES   OF  GEOLOGY." 


"  It  is  a  philosophy  which  never  rests— its  law  is  progress :  a  point  which  yesterday  was  invisible  a  its  goal 
today,  and  will  be  its  starting-post  to-morrow."  —  Edin.  Seo. 

"Part  of  the  present  Treatise  was  written  originally  in  the  form  of  a  supplement  to  my  former  work  enti- 
tled '  Principles  of  Geology,'  and  was  intended  for  the  use  of  those  students  who  found  certain  chapters  in 
the  'Principles'  obscure  and  difficult,  for  want  of  preliminary  information.  I  afterwards  considered  that  it 
would  not  be  incompatible  with  this  object  to  enlarge  the  '  Elements'  into  a  separate  and  independent  treatise, 

to  serve  as  an  Introduction  to  Geology  proper The  volume  now  offered  to  the  public,  is  neither 

an  epitome  of  the  'Principles,'  nor  an  abridgement  of  any  part  of  that  work."  —  Extract  from  the  Author** 
Preface. 


FIRST  AMERIC 
FROM  THE  FIRST  LONDON 


PHILADELPHIA: 

JAMES  KAY,  JUN.  AND  BROTHER,  122  CHESTNUT  STREET. 

PITTSBURGH:  C.  H.  KAY  &  CO. 

1839. 


iv  PREFACE. 

which  an  attempt  is  made  to  point  out  the  bearing  on  geology 
of  the  modern  changes  of  the  earth,  and  to  which  is  prefixed 
a  history  of  the  opinions  which  have  been  entertained  in  this 
science,  from  the  times  of  the  earliest  writers  to  the  present 
day. 

The  volume,  therefore,  now  offered  to  the  public,  is  neither 
an  epitome  of  the  Principles,  nor  an  abridgement  of  any  part 
of  that  work.  In  some  places,  where  I  thought  it  desirable 
to  incorporate  in  the  Elements  certain  passages  of  the  former 
work,  I  have  not  abridged  what  was  previously  written,  but 
have  expanded  it,  giving  fuller  explanations,  and  additional 
wood-cuts,  in  the  hope  of  rendering  it  more  intelligible  to  the 
beginner. 

>  Through  the  kindness  of  two  of  my  friends  I  have  been 
enabled  to  refer  frequently  to  two  works,  not  yet  before  the 
public,  Mr.  Darwin's  Journal  of  Travels  in  South  America, 
1832  to  1836,  &c.,  and  Mr.  Murchison's  Silurian  System  ; 
the  last  of  which  was  presented  to  me  complete,  with  thei  ex- 
ception of  the  maps  and  plates,  and  will  shortly  be  published. 
Mr.  Darwin's  Journal  was  finished,  and  ready  for  publica- 
tion, some  time  before  the  printing  of  my  MS.  had  begun,  but 
is  still  detained,  to  the  great  regret  of  the  scientific  world, 
because  it  is  to  form  part  of  a  larger  work,  including  an 
account  of  the  Surveys  of  Captains  King  and  FitzRoy,  in 
South  America. 


CONTENTS. 


PART  I. 


CHAPTER  I. 

ON  THE  FOUR  GREAT  CLASSES  OF  ROCKS — THE  AQUEOUS,  VOLCANIC, 
PLUTONIC,  AND  METAMORPHIC. 

Geology  defined — Successive  formation  of  the  earth's  crust — Classification  of 
rocks  according  to  their  origin  and  age  —  Aqueous  rocks  (p.  15.)  —  Their 
stratification  and  imbedded  fossils — Volcanic  rocks,  with  and  without  cones 
and  craters  (p.  18.)  —  Plutonic  rocks,  and  their  relation  to  the  volcanic  — 
Metamorphic  rocks,  and  their  probable  origin  (p.  21.) — The  term  primitive, 
why  erroneously  applied  to  the  crystalline  formations  (p.  23.)  —  Division  of 
the  work  into  two  parts ;  the  first  descriptive  of  rocks  without  reference  to 
their  age,  the  second  treating  of  their  chronology. 


CHAPTER  II. 

AQUEOUS    ROCKS — THEIR   COMPOSITION  AND    FORMS   OF  STRATIFICATION. 

Mineral  composition  of  strata  —  Arenaceous  rocks  —  Argillaceous  —  Calcareous 
—  Gypsum  —  Forms  of  stratification  (p.  28.)  —  Original  horizontally  —  thin 
ning  out  —  Diagonal  arrangement  (p.  31.)  —  Ripple  mark. 

CHAPTER  III. 

ARRANGEMENT   OF    FOSSILS   IN   STRATA — FRESHWATER   AND   MARINE. 

Successive  deposition  indicated  by  fossils  —  Limestones  formed  of  corals  and 
shells  —  Proofs  of  gradual  increase  of  strata  derived  from  fossils  —  Serpula 
attached  to  spatangus  (p.  37.)  —  Wood  bored  by  teredina  —  Tripoli  and  semi- 
opal  formed  of  infusoria  —  Chalk  derived  principally  from  organic  bodies 
(p.  41.)  —  Distinction  of  freshwater  from  marine  formations  —  Genera  of  fresh- 
water and  land  shells  —  Rules  for  recognizing  marine  testacea — Gyrogonite 
and  chara  (p.  48.) — Freshwater  fishes  —  Alternation  of  marine  and  fresh- 
water deposits  —  Lym-Fiord. 

A  *  (5) 


vi  CONTENTS. 

CHAPTER  IV. 

CONSOLIDATION   OF   STRATA   AND   PETRIFACTION   OF    FOSSILS. 

Chemical  and  mechanical  deposits  —  Cementing  together  of  particles  —  Hard, 
ening  by  exposure  to  air  (p.  52.) —  Concretionary  nodules  —  Consolidating 
effects  of  pressure  —  Mineralization  of  organic  remains  (p.  55.)  —  Impressions 
and  casts  how  formed — Fos^iTwood  —  Gb'ppert's  experiments  —  Precipitation 
of  stony  matter  most  rapid  where^  putrefaction  is  going  on  —  Source  of  lime 
in  solution  (p.  59.)  — Silex  derived  from  decomposition  of  felspar  —  Proofs  of 
the  lapidification  of  some  fossils  soon  after  burial,  of  others  when  much 
decayed. 

CHAPTER  V. 

ELEVATION   OF   STRATA    ABOVE   THE    SEA — HORIZONTAL   AND    INCLINED 
STRATIFICATION. 

Why  the  elevated  position  of  marine  strata  should  be  referred  to  the  rising  up 
of  the  land,  not  to  the  going  down  of  the  sea — Upheaval  of  extensive  masses 

•  of  horizontal  strata  (p.  63.)  —  Inclined  and  vertical  stratification  —  Anticlinal 
and  synclinal  lines  —  Examples  of  bent  strata  in  east  of  Scotland  (p.  66.)  — 
Theory  of  folding  by  lateral  movement  —  Dip  and  strike  (p.  69.)  —  Structure 
of  the  Jura — Rocks  broken  by  flexure  —  Inverted  position  of  disturbed  strata 
(p.  73.)  —  Unconformable  stratification  —  Fractures  of  strata  —  Polished  sur- 
faces—  Faults  —  Appearance  of  repeated  alternations  produced  by  them 
(p.  77.)  —  Origin  of  great  faults. 

CHAPTER  VI. 

DENUDATION,    AND   THE    PRODUCTION   OF   ALLUVIUM. 

Denudation  defined  —  Its  amount  equal  to  the  entire  mass  of  stratified  deposits 
in  the  earth's  crust  —  Horizontal  sandstone  denuded  in  Ross-shire  —  Levelled 
surface  of  countries  in  which  great  faults  occur  (p.  81.)  —  Connexion  of 
denudation  and  alluvial  formations  —  Alluvium,  how  distinguished  from  rocks 
in  situ  (p.  84.)  —  Ancient  alluviums  called  diluvium  —  Origin  of  these  — 
Erratic  blocks  and  accompanying  gravel  (p.  86.)  —  Theory  of  their  transporta- 
tion by  ice. 

CHAPTER  VII. 

VOLCANIC    ROCKS. 

Trap  rocks  —  Name,  whence  derived  —  Their  igneous  origin  at  first  doubted 
—  Their  general  appearance  and  character  —  Volcanic  cones  and  craters, 
how  formed  (p.  91.)  —  Mineral  composition  and  texture  of  volcanic  rocks  — 
Varieties  of  felspar  —  Hornblende  and  augite  — Isomorphism  (p.  94.)  — Rocks, 
how  to  be  studied  —  Basalt,  greenstone,  trachyte,  porphyry,  scoria,  amygda- 
loid (p.  96.)  lava,  tuff— Alphabetical  list,  and  explanation  of  names  and 
synonyms,  of  volcanic  rocks  (p.  99.)  —  Table  'of  the  analyses  of  minerals  most 
abundant  in  the  volcanic  and  hypogene  rocks. 


CONTENTS.  vii 

CHAPTER  VIII. 

VOLCANIC  ROCKS  —  continued. 

Trap  dikes  —  sometimes  project  —  sometimes  leave  fissures  vacant  by  decom- 
position—  Branches  and  veins  of  trap— Dikes  more  crystalline  in  the  centre 
(p.  105.)  —  Foreign  fragments  of  rock  imbedded  —  Strata  altered  at  or  near 
the  contact  —  Obliteration  of  organic  remains  —  Conversion  of  chalk  into 
marble — and  of  coal  into  coke  (p.  108.)  —  Inequality  in  the  modifying  influ- 
ence of  dikes  —  Trap  interposed  between  strata  —  Columnar  and  globular 
structure  (p.  110.)  —  Relation  of  trappean  rocks  to  the  products  of  active 
volcanos  (p.  114.)  —  Submarine  lava  and  ejected  matter  corresponds  gene- 
rally to  ancient  trap. 

CHAPTER  IX. 

PLUTONIC  ROCKS  —  GRANITE. 

General  aspect  of  granite  —  Decomposing  into  spherical  masses  —  Rude  colum- 
nar structure  —  Analogy  and  difference  of  volcanic  and  plutonic  formations 

—  Minerals  in  granite,  and  their  arrangement  —  Graphic  and  porphyritic 
granite    (p.  121.)  —  Occasional   minerals  —  Syenite  —  Syenitic,    talcose,  and 
schorly  granites  —  Eurite  —  Passage  of  granite  into  trap  —  Examples  near 
Christiania  and  in  Aberdeenshire  —  Analogy  in  composition  of  trachyte  and 
granite  —  Granite  veins  in  Glen  Tilt,  Cornwall,  the  Valorsine,  and  other 
countries  (p.  123.)  —  Different  composition  of  veins  from  main  body  of  granite 

—  Metalliferous  veins  in  strata  near  their  junction  with  granite  (p.  128.)  — 
Apparent  isolation  of  nodules  of  granite  —  Quartz  veins  —  Whether  plutonic 
rocks  are  ever  overlying  — Their  exposure  at  the  surface  due  to  denudation 
(p.  131.) 

CHAPTER  X. 

METAMORPHIC    ROCKS. 

General  character  of  metamorphic  rocks  —  Gneiss  —  Hornblende-schist  —  Mica- 
schist  —  Clay-slate  (p.  133.)  —  Quartzite  —  Chlorite-schist  —  Metamorphic 
limestone  —  Alphabetical  list  and  explanation  of  other  rocks  of  this  family 

—  Origin  of  the  metamorphic  strata  (p.  135.)  —  Their  stratification  is  real 
and  distinct  from  cleavage  —  On  joints  and  slaty  cleavage  (p.  138.)  —  Sup- 
posed causes  of  these  structures  —  how  far  connected  with  crystalline  action. 

CHAPTER  XI. 

METAMORPHIC  ROCKS  —  continued. 

Strata  near  some  intrusive  masses  of  granite  converted  into  rocks  identical 
with  different  members  of  the  metamorphic  series  —  Arguments  hence  de- 
rived as  to  the  nature  of  plutonic  action  (p.  146.)  —  Time  may  enable  this 
action  to  pervade  denser  masses  —  From  what  kinds  of  sedimentary  rock 
each  variety  of  the  metamorphic  class  may  be  derived  (p.  150.)  —  Certain 
objections  to  the  metamorphic  theory  considered. 


viii  CONTENTS. 


PART  II. 

CHAPTER  XII. 

ON  THE  DIFFERENT  AGES  OF  THE  FOUR  GREAT  CLASSES  OF  ROCKS. 

Aqueous,  plutonic,  volcanic,  and  metamorphic  rocks,  considered  chronologically 
—  Lehman's  division  into  primitive  and  secondary  —  Werner's  addition  of 
a  transition  class  —  Neptunian  theory  (p.  153.)  —  Hutton  on  igneous  origin 
of  granite  —  How  the  name  of  primary  was  still  retained  for  granite  —  The 
term  "  transition,"  why  faulty  —  The  adherence  to  the  old  chronological 
nomenclature  retarded  the  progress  of  geology  —  New  hypothesis  invented 
to  reconcile  the  igneous  origin  of  granite  to  the  notion  of  its  high  antiquity 
(p.  155.)  —  Explanation  of  the  chronological  nomenclature  adopted  in  this 
work,  so  far  as  regards  primary,  secondary,  and  tertiary  periods. 

CHAPTER  XIII. 

ON   THE    DIFFERENT    AGES    OF    THE    AQUEOUS    ROCKS. 

On  the  three  principal  tests  of  relative  age  —  superposition,  mineral  character, 
and  fossils  —  Change  of  mineral  character  and  fossils  in  the  same  continuous 
formation  —  Proofs  that  distinct  species  of  animals  and  plants  have  lived  at 
successive  periods  —  Test  of  age  by  included  fragments  (p.  162.)  —  Frequent 
absence  of  strata  of  intervening  periods  —  Principal  groups  of  strata  in  west- 
ern Europe  —  Tertiary  strata  separable  into  four  groups,  the  fossil  shells  of 
which  approach  nearer  to  those  now  living  in  proportion  as  the  formation 
is  more  modern  (p.  164.) — Terms  Eocene,  Miocene,  and  Pliocene  —  Identi- 
fications of  fossil  and  recent  shells  by  M.  Deshayes  —  Opinions  of  Dr.  Beck 
(p.  168.) 

CHAPTER  XIV. 

RECENT    AND    TERTIARY    FORMATIONS. 

How  to  distinguish  Recent  from  Tertiary  strata  —  Recent  and  Newer  Pliocene 
strata  near  Naples  —  near  Stockholm  and  Christiania  —  in  South  America,  on 
coasts  of  Chili  and  Peru  —  Rocks  of  Recent  period,  with  human  skeleton,  in 
Guadaloupe  (p.  171.)  —  Shells  of  living  species,  with  extinct  mammalia,  in 
loess  of  the  Rhine  —  Recent  and  newer  Pliocene  deposits  in  England  — 
Older  Pliocene  strata  in  England  —  Crag  —  Red  and  Coralline  crag  —  their 
fossils  in  part  distinct  (p.  175.)  —  their  strata  unconformable  —  belong  to  the 
same  period  —  London  clay  (p.  178.)  —  Its  shells  and  fish  imply  a  tropical 
climate  —  Tertiary  mammalia  —  Fossil  quadrumana. 


CONTENTS.  ix 

CHAPTER  XV. 

CRETACEOUS    GROUP. 

White  chalk  —  Its  marine  origin  shown  by  fossil  shells  —  Extinct  genera  of 
cephalopoda  —  Sponges  and  corals  in  the  chalk  —  No  terrestrial  or  fluviatile 
shells,  no  land  plants  —  Supposed  origin  of  white  chalk  from  decomposed 
corals  (p.  186.)  —  Single  pebbles,  whence  derived  —  Cretaceous  coral-reef  in 
Denmark  (p.  188.)  —  Maestricht  beds  and  fossils  —  Origin  of  flint  in  chalk  — 
Wide  area  covered  by  chalk  (p.  191.) — Green-sand  formation  and  fossils  — 
Origin  of — External  configuration  of  chalk  (p.  193.)  —  Outstanding  columns 
or  needles  —  Period  of  emergence  from  the  sea  —  Difference  of  the  chalk  of 
the  north  and  south  of  Europe  —  Hippurites  —  Nummulites  (p.  198.)  — 
Altered  lithological  character  of  cretaceous  formation  in  Spain  and  Greece  — 
Terminology. 

CHAPTER  XVI. 

WEALDEN    GROUP. 

The  Wealden,  including  the  Weald  clay,  Hastings  sand,  and  Purbeck  beds  — 
Intercalated  between  two  marine  formations  —  Fossil  shells  freshwater,  with 
a  few  marine — Cypris  —  Fish  —  Reptiles  (p.  202.)  —  Birds  —  Plants  —  Section 
showing  passage  of  Wealden  beneath  chalk  —  Junction  of  Wealden  and 
Oolite  —  Dirt-bed  (p.  205.)  —  Theory  of  gradual  subsidence  —  Proofs  that  the 
Wealden  strata,  notwithstanding  their  thickness,  may  have  been  formed  in 
shallow  water  (p.  208.)  —  Geographical  extent  of  Wealden  —  Bray  near 
Beauvais  —  Relation  of  the  Wealden  to  the  Lower  Green-sand  and  Oolite 
(p.  211.) 

CHAPTER  XVII. 

OOLITE    AND   LIAS. 

Subdivisions  of  the  Oolitic  group  —  Fossil  shells  —  Corals  in  the  calcareous 

divisions  only  —  Buried  forest  of  Encrinites  in  Bradford  clay  (p.  216.) 

Changes  in  organic  life  during  accumulation  of  Oolites  —  Characteristic 
fossils  —  Signs  of  neighbouring  land  and  shoals  (p.  221.)  —  Supposed  cetacea 
in  Oolite  •—  Oolite  of  Yorkshire  and  Scotland  (p.  223.) 

CHAPTER  XVIII. 

OOLITE  AND  LIAS  —  continued. 

Mineral    character  of  Lias  —  Name  of  Gryphite  limestone  —  Fossil  fish 

Ichthyodorulites  —  Reptiles  of  the  Lias  (p.  227.)  —  Ichthyosaur  and  Plesiosaur 

—  Newly-discovered  marine  Reptile  of  the  Galapagos  Islands  (p.  229.) 

Sudden  death  and  burial  of  fossil  animals  in  Lias  —  Origin  of  the  Oolite  and 
Lias,  and  of  altejnating  calcareous  and  argillaceous  formations  (p.  232.)  — 
Physical  geography  (p.  234.) —  Vales  of  clay  — Hills  and  escarpments  of 
limestone. 


x  CONTENTS. 

CHAPTER  XIX. 

NEW    RED    SANDSTONE    GROUP. 

Distinction  between  New  and  Old  Red  sandstone  —  Between  Upper  and  Lower 
New  Red  —  Muschelkalk  in  Germany  (p.  237.)  —  Fossil  plants  and  shells  of 
New  Red  Group,  entirely  different  from  Lias  and  Magnesian  limestone  — 
Lower  New  Red  and  Magnesian  limestone  (p.  239.)  —  Zechstein  in  Germany 
of  the  same  age  —  General  resemblance  between  the  organic  remains  of  the 
Magnesian  limestone  and  Carboniferous  strata  —  Origin  of  red  sand-stone  and 
red  marl  (p.  242.) 

CHAPTER  XX. 

THE  COAL,  OR  CARBONIFEROUS  GROUP. 

Carboniferous  strata  in  the  south-west  of  England  —  Superposition  of  Coal- 
measures  to  Mountain  limestone  —  Departure  from  this  type  in  north  of 
England  and  Scotland  —  Freshwater  strata  (p.  244.)  —  Intermixture  of  fresh- 
water and  marine  beds  —  Sauroidal  fish  —  Fossil  plants  (p.  247.)  —  Ferns  and 
Sigillariae  —  Lepidodendra  —  Calamites  (p.  250.)  —  Coniferae  —  Stigmarise. 

CHAPTER  XXI. 

CARBONIFEROUS    GROUP    Continued,    AND    OLD    RED    SANDSTONE. 

Corals  and  shells  of  the  Mountain  limestone  —  Hot  climate  of  the  Carboniferous 
period  inferred  from  the  marine  fossils  of  the  Mountain  limestone  and  the 
plants  of  the  Coal  (p.  254.)  —  Origin  of  the  Coal-strata — Contemporaneous 
freshwater  and  marine  deposits  —  Modern  analogy  of  strata  now  in  progress  in 
and  around  New  Zealand  —  Vertical  and  oblique  position  of  fossil  trees  in  the 
Coal  (p.  257.)  —  How  enveloped  —  How  far  they  prove  a  rapid  rate  of  depo- 
sition—Old Red  sandstone  (p.  262.)  — its  subdivisions  —  its  fossil  shells  and 
fish. 

CHAPTER  XXII. 

PRIMARY    FOSSILIFEROUS    STRATA. 

Primary  Fossiliferous  or  Transition  Strata  —  Term  "Grauwacke" —  Silurian 
Group  —  Upper  Silurian  and  Fossils  (p.  265.)  —  Lower  Silurian  and  Fossils  — 
Trilobites  (p.  267.)  —  Graptolites  —  Orthocerata  —  Occasional  horizontal! ty  of 
Silurian  Strata — Cambrian  Group  (p.  270.)  —  Endosiphonite. 

CHAPTER  XXIII. 

ON   THE    DIFFERENT    AGES    OF    THE^VOLCANIC    ROCKS. 

Tests  of  relative  age  of  volcanic  rocks  —  Test  by  superposition  and  intrusion  — 
By  alteration  of  rocks  in  contact  —  Test  by  organic  remains  (p.  273.)  —  Test 
of  age  by  mineral  character  —  Test  by  included  fragments  —  Volcanic  rocks 
of  the  Recent  and  Newer  Pliocene  periods  (p.  275.)  —  Miocene  —  Eocene  — 
Cretaceous  —  Oolitic  (p.  278.)  —  New  Red  sandstone  period  —  Carboniferous 
—  Old  Red  sandstone  period  —  Silurian  —  Upper  and  Lower  Cambrian  peri- 
ods (p.  280.)  —  Relative  ages  of  intrusive  traps. 


CONTENTS. 


CHAPTER  XXIV. 

ON  THE  DIFFERENT  AGES  OF  THE  PLUTONIC  ROCKS. 

Difficulty  in  ascertaining  the  precise  age  of  a  plutonic  rock  —  Test  of  age  by 
relative  position  —  Test  by  intrusion  and  alteration  —  Test  by  mineral  compo- 
sition—  Test  by  included  fragments  —  Recent  and  Pliocene  plutonic  rocks, 
why  invisible  (p.  283.) — Tertiary  plutonic  rocks  in  the  Andes  —  Granite 
altering  cretaceous  rocks  (p.  286.)  —  Granite  altering  Lias  in  the  Alps  and  in 
Sky  —  Granite  of  Dartmoor  altering  Carboniferous  strata  —  Granite  of  the  Old 
Red  sandstone  period —  Syenite  altering  Silurian  strata  in  Norway  (p.  289.)  — 
Blending  of  the  same  with  gneiss  —  Most  ancient  plutonic  rocks  —  Granite 
protruded  in  a  solid  form  (p.  291.)  —  On  the  probable  age  of  the  granite  of 
Arran  in  Scotland. 

CHAPTER  XXV. 

ON    THE    DIFFERENT    AGES    OF    THE    METAMORPHIC    ROCKS. 

Age  of  each  set  of  metamorphic  strata  twofold  (p.  294.)  —  Test  of  age  by  fossils 
and  mineral  character  not  available  —  Test  by  superposition  ambiguous  — 
Conversion  of  dense  masses  of  fossiliferous  strata  into  metamorphic  rocks  — 
Limestone  and  shale  of  Carrara  —  Metamorphic  strata  of  modern  periods  in 
the  Alps  of  Switzerland  and  Savoy  (p.  296.)  —  Why  the  visible  crystalline 
strata  are  none  of  them  very  modern  —  Order  of  succession  in  metamorphic 
rocks  (p.  299.)  —  Uniformity  of  mineral  character  —  Why  the  metamorphic 
strata  are  less  calcareous  than  the  fossiliferous  (p.  301.) 


Lately  Issued, 

BY    THE    SAME     PUBLISHERS, 

First  American,  from  the  Fifth  London  Edition,  Illustrated  by  226  Wood  Engravings, 
and  16  Plates  and  Maps  — 2  vols.  royal  8vo, 

PRINCIPLES  OF  GEOLOGY, 

BY  CHARLES  LYELL,  F.R.S., 

BEING 

AN    INQUIRY    HOW    FAR   THE    FORMER    CHANGES    OF    THE    EARTH'S    SUR- 
FACE ARE  REFERABLE  TO  CAUSES  NOW  IN  OPERATION,  ETC. 


The  following  Notices  may  serve  to  show  the  high  character  which  the  above 
work  enjoys,  as  the  Standard  Text  Book  for  advanced  Students  of  Geology,  both 
in  Great  Britain  and  the  United  States : — 

From  the  London  Quarterly  Review.—  '  We  hail  with  the  greatest  satisfaction  the  appear- 
ance of  Mr.  Lyell's  work,  which  henceforward,  we  can  hardly  doubt,  will  mark  the  begin- 
ning of  a  new  era  in  Geology.  The  title  of  the  book  shows  that  it  is  an  attempt  to  place 
the  study  of  the  science  on  its  true  basis— to  explain  the  former  changes  of  the  earth's  sur- 
face by  reference  to  causes  now  in  operation.  The  mode  in  which  this  undertaking  has  been 
executed,  is  most  satisfactory,  and  confirms  the  high  reputation  Mr.  Lyell  enjoys  for  zeal  and 
accuracy  in  observation,  and  an  intimacy  with  many  of  the  branches  of  science  and  natural 
history  which  bear  upon  Geology.  It  exhibits,  also,  together  with  much  literary  research  and 
elegance  of  language,  a  luminous  arrangement  and  powers  of  analytical  reasoning,  which 
we  should  be  glad  to  meet  with  more  frequently  in  the  contributions  to  our  scientific  know- 
ledge. Incorporated  with  his  arguments,  and  the  details  extracted  from  other  sources,  Mr. 
Lyell  has,  moreover,  communicated  a  great  body  of  original  observations  of  much  interest, 
collected  during  the  tours  he  has  recently  made  for  scientific  purposes  on  the  continent.- We 
cannot  but  express  our  obligations  to  him  for  the  great  addition  he  has  made  to  our  knowl- 
edge of  nature,  and  the  beneficial  influence  it  is  likely  to  have  in  communicating  a  right 
direction  and  a  philosophical  spirit  of  induction  to  geological  inquiry." 

From  Silliman's  American  Journal  of  Science  and  Arts.—"  The  rapidity  with  which  new 
editions  of  this  excellent  Work  have  appeared,  sufficiently  evinces  the  estimation  in  which 
it  is  held.  We  are  indebted  to  the  industry,  good  judgment,  and  great  science  of  Mr.  Lyell, 
for  a  lucid  and  highly  interesting  exhibition  of  facts,  and  for  a  logical  and  candid  discussion 
of  principles.  He  has  done  much  to  recal  Geologists  from  extravagant  speculations,  and  to 
allure  them  back  to  a  course  of  strict  induction  ;  thus  placing  Geology  side  by  side  with  the 
other  sciences  of  observation.  The  publication  of  Mr.  Lyell's  Work  forms  a  new  era  in 
Geology  ;  it  must  be  studied  by  every  person  who  would  be  acquainted  with  the  present  im- 
proved state  of  the  science  ;  and  happily  the  study  will  prove  no  task  ;  for  the  lucid  and 
beautiful  style  of  the  author,  embellished  by  occasional  classical  flowers,  gives  this  Work 
almost  as  peculiar  a  character,  as  its  novel  philosophy." 

From  the  Transactions  of  the  Geological  Society  of  Pennsylvania.— "It  can  scarcely  be 
necessary  to  say  any  thing  in  praise  of  this  Work.  Its  appearance  will  always  form  an 
epoch  in  the  history  of  Geology.  Up  to  that  time  the  doctrine  which  assumed  the  causes  of 
changes,  whether  of  a  destroying  or  productive  character,  actually  in  progress  on  the  surface 
of  the  globe,  to  be  utterly  inadequate  to  explain,  scarcely  even  to  illustrate,  the  earlier  changes 
of  which  that  surface  exhibits  such  striking  traces,  held  almost  undisputed  sway  in  the  geologi- 
cal circles.  Mr.  Lyell.  applying  himself  to  the  elucidation  of  the  existing  causes  of  change, 
and  their  probable  influence  on  the  older  geological  formations,  with  an  industry  and  research 
which  are  joined  to  the  happiest  powers  of  description  and  command  of  language,  has  pro- 
duced a  Work  not  only  of  the  highest  interest  to  the  scientific  world,  but  of  the  most  popular 
and  fascinating  nature  to  the  general  reader." 

(12) 


ELEMENTS  OF  GEOLOGY. 


PART  I. 


CHAPTER  I. 

ON  THE  FOUR  GREAT  CLASSES  OF  ROCKS — THE  AQUEOUS,  VOLCANIC, 
PLUTONIC,  AND  METAMORPHIC. 

Geology  defined  —  Successive  formation  of  the  earth's  crust — Classification 
of  rocks  according  to  their  origin  and  age — Aqueous  rocks — Their  stratification 
and  imbedded  fossils — Volcanic  rocks,  with  and  without  cones  and  craters — 
Plutonic  rocks,  and  their  relation  to  the  volcanic  —  Metamorphic  rocks,  and 
their  probable  origin — The  term  primitive,  why  erroneously  applied  to  the 
crystalline  formations  —  Division  of  the  work  into  two  parts ;  the  first  descrip- 
tive of  rocks  without  reference  to  their  age,  the  second  treating  of  their  chro- 
nology. 

OF  what  materials  is  the  earth  composed,  and  in  what  man- 
ner are  these  materials  arranged  1  These  are  the  inquiries  with 
which  Geology  is  occupied,  a  science  which  derives  its  name 
from  the  Greek,  y»?,  ge^  the  earth,  and  toyoj,  logos,  a  discourse. 
Such  investigations  appear,  at  first  sight,  to  relate  exclusively  to 
the  mineral  kingdom,  and  to  the  various  rocks,  soils,  and  metals, 
which  occur  upon  the  surface  of  the  earth,  or  at  various  depths 
beneath  it.  But,  in  pursuing  these  researches,  we  soon  find  our- 
selves led  on  to  consider  the  successive  changes  which  have 
taken  place  in  the  former  state  of  the  earth's  surface  and  inte- 
rior, and  the  causes  which  have  given  rise  to  these  changes;  and, 
what  is  still  more  singular  and  unexpected,  we  soon  become 
engaged  in  researches  into  the  history  of  the  animate  creation, 
or  of  the  various  tribes  of  animals  and  plants  which  have,  at  dif- 
ferent periods  of  the  past,  inhabited  the  globe. 

All  are  aware  that  the  solid  parts  of  the  earth  consist  of  dis- 
tinct  substances,  such  as  clay,  chalk,  sand,  limestone,  coal,  slate, 
granite,  and  the  like ;  but  previously  to  observation  it  is  com- 
monly imagined  that  all  these  had  remained  from  the  first  in  the 
state  in  which  we  now  see  them, — that  they  were  created  in  their 
present  form,  and  in  their  present  position.  Geologists  have 
B  (13) 


14       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Successive  Formation  of  the  Earth's  Crust. 

come  to  a  different  conclusion.  They  have  discovered  proofs 
that  the  external  parts  of  the  earth  were  not  all  produced  in  the 
beginning  of  things,  in  the  state  in  which  we  now  behold  them, 
nor  in  an  instant  of  time.  On  the  contrary,  they  have  acquired 
their  actual  configuration  and  condition  gradually,  under  a  great 
variety  of  circumstances,  and  at  successive  periods,  during  each , 
of  which  distinct  races  of  living  beings  have  flourished  on  the 
land  and  in  the  waters,  the  remains  of  these  creatures  still  lying 
buried  in  the  crust  of  the  earth. 

By  the  "  earth's  crust,"  is  meant  that  small  portion  of  the 
exterior  of  our  planet  which  is  accessible  to  human  observation. 
It  comprises  not  merely  all  of  which  the  structure  is  laid  open 
in  mountain  precipices,  or  in  cliffs  overhanging  the  river  or  the 
sea,  or  whatever  the  miner  may  reveal  in  artificial  excavations ; 
but  the  whole  of  that  outer  covering  of  the  planet  on  wliich  we 
are  enabled  to  reason  by  observations  made  at  or  near  the  sur- 
face. These  reasonings  may  extend  to  a  depth  of  several  miles, 
perhaps  ten  miles ;  but  even  then  it  may  be  said,  that  such  a 
thickness  is  no  more  than  ^^th  Part  °f  tne  distance  from  the 
surface  to  the  centre.  The  remark  is  just ;  but  although  the 
dimensions  of  such  a  crust  are,  in  truth,  insignificant  when  com- 
pared to  the  entire  globe,  yet  they  are  vast  and  of  magnificent 
extent  in  relation  to  man,  and  to  the  organic  beings  which  peo- 
ple our  globe.  Referring  to  this  standard  of  magnitude,  the 
geologist  may  admire  the  ample  limits  of  his  domain,  and  admit, 
at  the  same  time,  that  not  only  the  exterior  of  the  planet,  but  the 
entire  earth,  is  but  an  atom  in  the  midst  of  the  countless  worlds 
surveyed  by  the  astronomer. 

Now  the  materials  of  this  crust  are  not  thrown  together  con- 
fusedly, but  distinct  mineral  masses,  called  rocks,  are  found  to 
occupy  definite  spaces,  and  to  exhibit  a  certain  order  of  arrange- 
ment. The  term  rock  is  applied  indifferently  by  geologists  to 
all  these  substances,  whether  they  be  soft  or  stony,  for  clay  and 
sand  are  included  in  the  term,  and  some  have  even  brought  peat 
under  this  denomination.  Our  older  writers  endeavoured  to 
avoid  offering  such  violence  to  our  language,  by  speaking  of  the 
component  materials  of  the  earth  as  consisting  of  rocks  and  soils. 
But  there  is  often  so  insensible  a  passage  from  a  soft  and  inco- 
herent state  to  that  of  stone,  that  geologists  of  all  countries  have 
found  it  indispensable  to  have  one  technical  term  to  include  both, 
and  in  this  sense  we  find  roche  applied  in  French,  rocca  in  Ita- 
lian, and  felsart  in  German. 

The  beginner,  however,  must  constantly  bear  in  mind,  that  the 
term  rock  by  no  means  implies  that  a  mineral  mass  is  in  an 
indurated  or  stony  condition. 


PART  I.     CHAPTER  I.  15 

Classification  of  Rocks Aqueous  Rocks. 

In  order  to  classify  the  various  rocks  which  compose  the 
earth's  crust,  it  is  found  most  convenient  to  refer,  in  the  first 
place,  to  their  origin,  and  in  the  second  to  their  age.  I  shall 
therefore  begin  by  endeavouring  briefly  to  explain  to  the  student 
how  all  rocks  may  be  divided  into  four  great  classes  by  refer- 
ence to  their  different  origin,  or,  in  other  words,  by  reference  to 
the  different  circumstances  and  causes  by  which  they  have  been 
produced. 

The  first  two  divisions,  which  will  at  once  be  understood  as 
natural,  are  the  aqueous  and  volcanic,  or  the  products  of  watery 
and  those  of  igneous  action. 

Aqueous  rocks. — The  aqueous  rocks,  sometimes  called  the 
sedimentary,  or  fossiliferous,  cover  a  larger  part  of  the  earth's 
surface  than  any  others.  These  rocks  are  stratified,  or  divided 
into  distinct  layers,  or  strata.  The  term  stratum  means  simply 
a  bed,  or  any  thing  spread  out  or  strewed  over  a  given  surface ; 
and  we  infer  that  these  strata  have  been  generally  spread  out  by 
the  action  of  water,  from  what  we  daily  see  taking  place  near 
the  mouths  of  rivers,  or  on  the  land  during  temporary  inunda- 
tions. For,  whenever  a  running  stream,  charged  with  mud  or 
sand,  has  its  velocity  checked,  as  when  it  enters  a  lake  or  sea, 
or  overflows  a  plain,  the  sediment,  previously  held  in  suspension 
by  the  motion  of  the  water,  sinks,  by  its  own  gravity,  to  the 
bottom.  In  this  manner  layers  of  mud  and  sand  are  thrown 
down  one  upon  another. 

If  we  drain  a  lake  which  has  been  fed  by  a  small  stream,  we 
frequently  find  at  the  bottom  a  series  of  deposits,  disposed  with 
considerable  regularity,  one  above  the  other;  the  uppermost, 
perhaps,  may  be  a  stratum  of  peat,  next  below  a  more  dense  and 
solid  variety  of  the  same  material ;  still  lower  a  bed  of  laminated 
shell-marl,  alternating  with  peat  or  sand,  and.  then  other  beds  of 
marl,  divided  by  layers  of  clay.  Now  if  a  second  pit  be  sunk 
through  the  same  continuous  lacustrine  formation,  at  some  dis- 
tance from  the  first,  we  commonly  meet  with  nearly  th^.  same 
series  of  beds,  yet  with  slight  variations ;  some,  for  example,  of 
the  layers  of  sand,  clay,  or  marl,  may  be  wanting,  one  or  more 
of  them  having  thinned  out  and  given  place  to  others,  or  some- 
times one  of  the  masses  first  examined  is  observed  to  increase  in 
thickness  to  the  exclusion  of  other  beds. 

The  term  "formation"  which  I  have  used  in  the  above 
explanation,  expresses  in  geology  any  assemblage  of  rocks  which 
have  some  character  in  common,  whether  of  origin,  age,  or 
/  composition.  Thus  we  speak  of  stratified  and  unstratified,  fresh- 
water and  marine,  aqueous  and  volcanic,  ancient  and  modern, 
metalliferous  and  non-metalliferous  formations. 


16       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Aqueous  Origin  of  certain  Rocks. 

In  the  estuaries  of  large  rivers,  such  as  the  Ganges  and  the 
Mississippi,  we  may  observe,  at  low  water,  phenomena  analogous 
to  those  of  the  drained  lakes  above  mentioned,  but  on  a  grander 
scale,  and  extending  over  areas  several  hundred  miles  in  length 
and  breadth.  When  the  periodical  inundations  subside,  the 
river  hollows  out  a  channel  to  the  depth  of  many  yards  through 
horizontal  beds  of  clay  and  sand,  the  ends  of  which  are  seen 
exposed  in  perpendicular  cliffs.  These  beds  vary  in  colour,  and 
are  occasionally  characterized  by  containing  drift-wood  or  shells. 
The  shells  may  belong  to  species  peculiar  to  the  river,  but  are 
sometimes  those  of  marine  testacea,  washed  into  the  mouth  of 
the  estuary  during  storms. 

The  annual  floods  of  the  Nile  in  Egypt  are  well  known,  and 
the  fertile  deposit  of  mud  which  they  leave  on  the  plains.  This 
mud  is  stratified,  the  thin  layer  thrown  down  in  one  season 
differing  slightly  in  colour  from  that  of  a  previous  year,  and 
being  separable  from  it,  as  has  been  observed  in  excavations  at 
Cairo,  and  other  places.* 

When  beds  of  sand,  clay,  and  marl,  containing  shells  and 
vegetable  matter,  are  found  arranged  in  the  same  manner  in  the 
interior  of  the  earth,  we  ascribe  to  them  a  similar  origin ;  and 
the  more  we  examine  their  characters  in  minute  detail,  the  more 
exact  do  we  find  the  resemblance.  Thus,  for  example,  at  vari- 
ous heights  and  depths  in  the  earth,  and  often  far  from  seas, 
lakes,  and  rivers,  we  meet  with  layers  of  rounded  pebbles  com- 
posed of  different  rocks  mingled  together.  They  are  like  the 
pebbles  formed  in  the  beds  of  torrents  and  rivers,  which  are  car- 
ried down  into  the  sea  wherever  these  descend  from  high  grounds 
bordering  a  coast.  There  the  gravel  is  spread  out  by  the  waves 
and  currents  of  the  ocean  over  a  considerable  space ;  but  during 
seasons  of  drought  the  torrents  and  rivers  are  nearly  dry,  and 
have  only  power  to  convey  fine  sand  or  mud  into  the  sea.  Hence, 
alternate  layers  of  gravel  and  fine  sediment  accumulate  under 
water,  and  such  alternations  are  found  by  geologists  in  the  inte- 
rior of  every  continent.f 

If  a  stratified  arrangement,  and  the  rounded  forms  of  pebbles, 
are  alone  sufficient  to  lead  us  to  the  conclusion  that  certain  rocks 
originated  under  water,  this  opinion  is  farther  confirmed  by  the 
distinct  and  independent  evidence  of  ^fossils,  so  abundantly 
included  in  the  earth's  crust.  By  a  fossil  is  meant  any  body,  or 
the  traces  of  the  existence  of  any  body,  whether  animal  or  vege- 

*  See  Silliman's  Amer.  Journ.  of  Sci.  vol.  xxviii.  1835 ;  also  Principles  of 
Geology,  Index,  "  Nile,  "  Rivers,"  &c. 

t  See  Principles  of  Geology,  by  the  author ;  refer  to  « Magnan,'  and  '  Conglo- 
merates, in  the  Index. 


PART  I.     CHAPTER  I.  17 


Aqueous  Origin  of  certain  Rocks. 


table,  which  has  been  buried  in  the  earth  by  natural  causes. 
Now  the  remains  of  animals,  especially  of  aquatic  species,  are 
found  almost  everywhere  imbedded  in  stratified  rocks.  Shells 
and  corals  are  the  most  frequent,  and  with  them  are  often  asso- 
ciated the  bones  and  teeth  of  fish,  fragments  of  wood,  impres- 
sions of  leaves,  and  other  organic  substances.  Fossil  shells  of 
forms  such  as  now  abound  in  the  sea,  are  met  with  far  inland, 
both  near  the  surface  and  at  all  depths  below  it,  as  far  as  the 
miner  can  penetrate.  They  occur  at  all  heights  above  the  level 
of  the  ocean,  having  been  observed  at  an  elevation  of  from  8000 
to  9000  feet  in  the  Alps  and  Pyrenees,  more  than  13,000  feet 
high  in«the  Andes,  and  above  15,000  feet  in  the  Himalayas. 

These  shells  belong  mostly  to  marine  testacea,  but  in  some 
places  exclusively  to  forms  characteristic  of  lakes  and  rivers. 
Hence,  we  conclude  that  some  ancient  strata  were  deposited  at 
the  bottom  of  the  sea,  while  others  were  formed  in  lakes  and 
estuaries. 

When  geology  was  first  cultivated,  it  was  a  general  belief  that 
these  marine  shells  and  other  fossils  were  the  effects  and  proofs  of 
the  general  deluge.  But  all  who  have  carefully  investigated  the 
phenomena  have  long  rejected  this  doctrine.  A  transient  flood 
might  be  supposed  to  leave  behind  it,  here  and  there  upon  the 
surface,  scattered  heaps  of  mud,  sand,  and  shingle,  with  shells 
confusedly  intermixed ;  but  the  strata  containing  fossils  are  not 
superficial  deposits,  and  do  not  cover  the  earth,  but  constitute  the 
entire  mass  of  mountains.  It  has  been  also  the  favourite  notion 
of  some  modern  writers,  who  are  aware  that  fossil  bodies  cannot 
all  be  referred  to  the  deluge,  that  they,  and  the  strata  in  which 
they  are  entombed,  may  have  been  deposited  in  the  bed  of  the 
ocean  during  a  period  of  several  thousand  years  which  inter- 
vened between  the  creation  of  man  and  the  deluge.  They  ima- 
gine that  the  antediluvian  bed  of  the  ocean,  after  having  been 
the  receptacle  of  many  stratified  deposits,  became  converted,  at 
the  time  of  the  flood,  into  the  lands  which  we  inhabit,  and  that 
the  ancient  continents  were  at  the  same  time  submerged,  and  be- 
came the  bed  of  the  present  sea.  This  hypothesis,  however 
preferable  to  the  diluvial  theory,  as  admitting  that  all  fossiliferous 
strata  were  slowly  and  successively  thrown  down  from  water,  is 
yet  wholly  inadequate  to  explain  the  repeated  revolutions  which 
the  earth  has  undergone,  and  the  signs  which  the  existing  con- 
tinents exhibit,  in  most  regions,  of  having  emerged  from  the 
ocean  at  an  era  far  more  remote  than  four  thousand  years  from 
the  present  time.  It  will  also  be  seen  in  the  sequel,  that  many 
distinct  sets  of  sedimentary  strata,  each  several  hundreds  or 
thousands  of  feet  thick,  are  piled  one  upon  the  other  in  the  earth's 

B  * 


18       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Volcanic  Rocks Volcanic  Origin  of  certain  Rocks. 

crust,  each  containing  their  peculiar  fossil  animals  and  plants, 
which  are  distinguishable,  with  few  exceptions,  from  species  now 
living.  The  mass  of  some  of  these  strata  consists  almost  entirely 
of  corals,  others  are  made  up  of  shells,  others  of  plants  turned 
into  coal,  while  some  are  without  fossils.  In  one  set  of  strata 
the  species  of  fossils  are  marine ;  in  another,  placed  immediately 
above  or  below,  they  as  clearly  prove  that  the  deposit  was  form- 
ed in  an  estuary  or  lake.  When  the  student  has  more  fully  ex- 
amined into  these  appearances,  he  will  become  convinced  that 
the  time  required  for  the  origin  of  the  actual  continents  must  have 
been  far  greater  than  that  which  is  conceded  by  the  theory  above 
alluded  to,  and  that  no  one  universal  and  sudden  conversion  of 
sea  into  land  will  account  for  geological  appearances. 

We  have  now  pointed  out  one  great  class  of  rocks,  which, 
however  they  may  vary  in  mineral  composition,  colour,  grain, 
or  other  characters,  external  and  internal,  may  nevertheless  be 
grouped  together  as  having  a  common  origin.  They  have  all 
been  formed  under  water,  in  the  same  manner  as  sand,  mud, 
shingle,  banks  of  shells,  coral,  and  the  like,  and  are  characterized 
by  stratification  or  fossils,  or  by  both. 

Volcanic  rocks. — The  division  of  rocks  which  we  may  next 
consider,  are  the  volcanic,  or  those  which  have  been  produced, 
whether  in  ancient  or  modern  times,  not  by  water,  but  by  the 
action  of  fire,  or  subterranean  heat.  These  rocks  are  for  the 
most  part  unstratified,  and  are  devoid  of  fossils.  They  are  more 
partially  distributed  than  aqueous  formations,  at  least  in  respect 
to  horizontal  extension. ;  Among  those  parts  of  Europe  where 
they  exhibit  characters  not  to  be  mistaken,  I  may  mention  not 
only  Sicily  and  the  country  round  Naples,  but  Auvergne,  Velay, 
and  Vivarais,  now  the  departments  of  Puy  de  Dome,  Haute 
Loire,  and  Ardeche,  towards  the  centre  and  south  of  France,  in 
which  we  find  several  hundred  conical  hills,  having  the  forms  of 
modern  volcanos,  with  craters  more  or  less  perfect  on  many  of 
their  summits.  These  cones  are  composed,  moreover,  of  lava, 
sand,  and  ashes,  similar  to  those  of  active  volcanos.  Streams  of 
lava  may  sometimes  be  traced  proceeding  from  the  cones  into 
the  adjoining  valleys,  where  they  choke  up  the  ancient  channels 
of  rivers  with  solid  rock,  in  the  same  manner  as  some  modern 
flows  of  lava  in  Iceland  have  been  known  to  do,  the  rivers  either 
flowing  beneath,  or  cutting  out  a  narrow  passage  on  one  side  of 
the  lava.  Although  none  of  these  French  volcanos  have  been 
in  activity  within  the  period  of  history  or  tradition,  their  forms 
are  often  very  perfect.  Some,  however,  have  been  compared  to 
the  mere  skeletons  of  volcanos,  the  rains  and  torrents  having 
washed  their  sides,  and  removed  all  the  loose  sand  and  scoriae, 


PART  I.     CHAPTER  I.  10 

Volcanic  Origin  of  certain  Rocks. 

leaving  only  the  harder  and  more  solid  materials.  By  this  ero- 
"  sion,  and  by  earthquakes,  their  internal  structure  has  occasion- 
ally been  laid  open  to  view,  in  fissures  and  ravines  ,•  and  we  then 
behold  not  only  many  successive  beds  and  masses  of  porous  lava, 
sand,  and  scoriae,  but  also  perpendicular  walls,  or  dikes,  as  they 
are  called,  of  volcanic  rock,  cutting  through  the  other  materials. 
Such  dikes  are  also  observed  in  the  structure  of  Vesuvius,  Etna, 
and  other  active  volcanos.  They  have  been  formed  by  the 
pouring  of  melted  matter,  whether  from  above  or  below,  into 
open  fissures,  and  they  commonly  traverse  deposits  of  volcanic 
tuff.)  a  substance  produced  by  the  showering  down  from  the  air, 
or  incumbent  waters,  of  sand  and  cinders,  first  shot  up  from  the 
interior  of  the  earth  by  explosions  of  volcanic  gases. 

Besides  the  parts  of  France  above  alluded  to,  there  are  other 
countries,  as  the  north  of  Spain,  the  south  of  Sicily,  the  Tuscan 
territory  of  Italy,  the  lower  Rhenish  provinces,  and  Hungary, 
where  spent  volcanos  may  be  seen  with  cones,  craters,  and  often 
accompanying  lava-streams. 

There  are  also  other  rocks  in  England,  Scotland,  Ireland,  and 
almost  every  country  in  Europe,  which  we  infer  to  be  of  igneous 
origin,  although  they  do  not  form  hills  with  cones  and  craters. 
Thus,  for  example,  we  feel  assured"  that  the  rock  of  Staffa,  and 
that  of  the  Giant's  Causeway,  called  basalt,  is  volcanic,  because 
it  agrees  in  its  columnar  structure  and  mineral  composition  with 
streams  of  lava  which  we  know  to  have  flowed  from  the  craters 
of  volcanos.  We  find  also  similar  basaltic  rocks  associated  with 
beds  of  tuff  in  various  parts  of  the  British  Isles,  and  forming  dikes, 
such  as  have  been  spoken  of;  and  some  of  the  strata  through 
which  these  dikes  cut  are  occasionally  altered  at  the  point  of 
contact,  as  if  they  had  been  exposed  to  the  intense  heat  of  melted 
matter. 

The  absence  of  cones  and  craters,  and  long  narrow  streams 
of  lava,  in  England  and  elsewhere,  is  principally  attributed  by 
geologists  to  the  eruptions  having  been  formerly  submarine,  just 
as  a  considerable  proportion  of  volcanos  in  our  own  times  burst 
out  beneath  the  sea.  But  this  question  must  be  enlarged  upon 
more  fully  in  the  chapters  on  Igneous  Rocks,  in  which  it  will 
also  be  shown,  that  as  different  sedimentary  formations,  contain- 
ing each  their  characteristic  fossils,  have  been  deposited  at  suc- 
cessive periods,  so  also  volcanic  sand  and  scoriae  have  been 
thrown  out,  and  lavas  have  flowed  over  the  land  or  bed  of  the 
sea,  at  many  different  epochs,  or  have  been  injected  into  fissures ; 
so  that  the  igneous  as  well  as  the  aqueous  rocks  may  be  classed 
as  a  chronological  series  of  monuments,  throwing  light  on  a 
succession  of  events  in  the  history  of  the  earth. 


20       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Plutonic  Rocks  and  their  Origin. 


Plutonic  rocks. — We  have  now  therefore  pointed  out  the  ex- 
istence of  two  distinct  orders  of  mineral  masses,  the  aqueous  and 
the  volcanic :  but  if  we  examine  a  large  portion  of  a  continent, 
especially  if  it  contain  within  it  a  lofty  mountain  range,  we  rarely 
fail  to  discover  two  other  classes  of  rocks,  very  distinct  from 
either  of  those  above  alluded  to,  and  which  we  can  neither  assimi- 
late to  deposits  sucli  as  are  now  accumulated  in  lakes  or  seas, 
nor  to  those  generated  by  ordinary  volcanic  action.  •-  The  mem- 
bers of  both  these  divisions  of  rocks  agree  in  being  highly  crys- 
talline and  destitute  of  organic  remains/;  The  rocks  of  one  divi- 
sion have  been  called  plutonic,  comprehending  alt  the  granites 
and  certain  porphyries,  which  are  nearly  allied  in  some  of  their 
characters  to  volcanic  formations.  <  The  members  of  the  other 
class  are  stratified  and  often  slaty,  and  have  been  called  by  some 
the  crystalline-schists.  In  these  are  included  gneiss,  micaceous- 
schist  (or  mica-slate,)  hornblende-schist,  statuary  marble,  the 
finer  kinds  of  roofing  slate,  and  other  rocks  afterwards  to  be 
described. 

As  it  is  admitted  that  nothing  strictly  analogous  to  these  crys- 
talline productions  can  now  be  seen  in  the  progress  of  formation 
on  the  earth's  surface,  it  will  naturally  be  asked,  on  what  data 
we  can  find  a  place  for  them  in  a  system  of  classification  founded 
on  the  origin  of  rocks.  First  then,  in  regard  to  the  plutonic 
class,  a  passage  has  been  traced  from  various  kinds  of  granite 
into  different  varieties  of  rocks  decidedly  volcanic;  so  that  if  the 
latter  are  of  igneous  origin,  it  is  scarcely  possible  to  refuse  to 
admit  that  the  granites  are  so  likewise.  Secondly,  large  masses 
of  granite  are  found  to  send  forth  dikes  and  veins  into  the  con- 
tiguous strata,  very  much  in  the  same  way  as  lava  and  volcanic 
matter  penetrate  aqueous  deposits,  both  the  massive  granite  and 
the  veins  causing  changes  analogous  to  those  which  lava  and 
volcanic  gases  are  known  to  produce.  But  the  plutonic  rocks 
differ  from  the  volcanic,  not  only  by  their  more  crystalline  tex- 
ture, but  also  by  the  absence  of  tuffs  and  breccias,  which  are  the 
products  of  eruptions  at  the  earth's  surface/1  They  differ  also  by 
the  absence  of  pores  or  cellular  cavities,  which  the  entangled  gases 
give  rise  to  in  ordinary  lava.  From  these  and  other  peculiari- 
ties it  has  been  inferred,  that  the  granites  have  been  formed  at 
great  depths  in  the  earth,  and  have  cooled  and  crystallized  slowly 
under  enormous  pressure  where  the  contained  gases  could  not 
expand.  The  volcanic  rocks,  on  the  contrary,  although  they 
also  have  risen  up  from  below,  have  cooled  from  a  melted  state 
more  rapidly  upon  or  near  the  surface.  From  this  hypothesis 
of  the  great  depth  at  which  the  granites  originated,  has  been  de- 
rived the  name  of  "  Plutonic  rocks,"  which  they  have  received 


PART  I.    CHAPTER  I.  21 


Metamorphic  Rocks 


to  distinguish  them  from  the  volcanic.  The  beginner  will  easily 
conceive  that  the  influence  of  subterranean  heat  may  extend 
downwards  from  the  crater  of  every  active  volcano  to  a  great 
depth  below,  perhaps  several  miles  or  leagues  (see  Frontispiece,) 
and  the  effects  which  are  produced  deep  in  the  bowels  of  the 
earth  may,  or  rather  must  be  distinct ;  so  that  volcanic  and  plu- 
tonic  rocks,  each  different  in  texture,  and  sometimes  even  in 
composition,  may  originate  simultaneously,  the  one  at  the  sur- 
face, the  other  far  beneath  it. 

Although  granite  has  often  pierced  through  other  strata,  it 
has  rarely,  if  ever,  been  observed,  to  rest  upon  them  as  if  it  had 
overflowed.  But  as  this  is  continually  the  case  with  the  volca- 
nic rocks,  they  have  been  styled  from  this  peculiarity,  "  overly- 
ing" by  Dr.  MacCulloch ;  and  Mr.  Necker  has  proposed  the 
term  "  underlying "  for  the  granites,  to  designate  the  opposite 
mode  in  which  they  almost  invariably  present  themselves. 

Metamorphic  rocks. — The  fourth  and  last  great  division  of 
rocks  are  the  crystalline  strata  or  schists,  called  gneiss,  mica- 
schist,  clay-slate,  chlorite-schist,  marble,  and  the  like,  the  origin 
of  which  is  more  doubtful  than  that  of  the  other  three  classes. 
They  contain  no  pebbles  or  sand  or  scoriae,  or  angular  pieces  of 
imbedded  stone,  and  no  traces  of  organic  bodies,  and  they  are 
often  as  crystalline  as  granite,  yet  are  divided  into  beds,  corre- 
sponding in  form  to  those  of  sedimentary  formations,  and  are 
therefore  said  to  be  stratified.  The  beds  sometimes  consist  of  an 
alternation  of  substances  varying  in  colour,  composition,  and 
thickness,  precisely  as  we  see  in  stratified  fossil  iferous  deposits. 
According  to  the  theory  which  I  adopt  as  most  probable,  and 
which  will  be  afterwards  more  fully  explained,  the  materials  of 
these  strata  were  originally  deposited  from  water  in  the  usual 
form  of  sediment,  but  they  were  subsequently  altered  by  subter- 
ranean heat,  so  as* to  assume  a  new  texture.  It  is  demonstrable, 
in  some  cases  at  least,  that  such  a  complete  conversion  has 
actually  taken  place.  I  have  already  remarked  that  alterations, 
such  as  might  be  produced  by  intense  heat,  are  observed  in  strata 
near  their  contact  with  veins  and  dikes  of  volcanic  rocks. 
These,  however,  are  on  a  small  scale ;  but  a  similar  influence 
has  been  exerted  much  more  powerfully  in  the  neighbourhood 
of  plutonic  rocks  under  different  circumstances,  and  perhaps  in 
combination  with  other  causes.  The  effects  thereby  superin- 
duced on  fossiliferous  strata  have  sometimes  extended  to  a  dis- 
tance of  a  quarter  of  a  mile  from  the  point  of  contact.  Through- 
out the  greater  part  of  this  space  the  fossiliferous  beds  have 
exchanged  an  earthy  for  a  highly  crystalline  texture,  and  have 
lost  all  traces  of  organic  remains.  Thus,  for  example,  dark 


22       lAELL'S  ELEMENTS  OF  GEOLOGY. 


Metamorphic  Rocks  and  their  Origin. 


limestones,  replete  with  shells  and  corals,  are  turned  into  white 
statuary  marble,  and  hard  clays  into  slates  called  mica-schist  and 
hornblende-schist,  all  signs  of  organic  bodies  having  been  oblite- 
rated. 

Although  we  are  in  a  great  degree  ignorant  of  the  precise 
nature  of  the  influence  here  exerted,  yet  it  evidently  bears  some 
analogy  to  that  which  volcanic  heat  and  gases  are  capable  of 
producing ;  and  the  action  may  be  conveniently  called  plutonic, 
because  it  appears  to  have  been  developed  in  those  regions  where 
plutonic  rocks  are  generated,  and  under  similar  circumstances 
of  pressure  and  depth  in  the  earth.  Whether  electricity  or  any 
other  causes  have  co-operated  with  heat  to  produce  this  influ- 
ence, may  be  matter  of  speculation,  but  the  plutonic  influence 
has  sometimes  pervaded  entire  mountain  masses  of  strata.  The 
phenomena,  therefore,  being  sometimes  on  so  grand  a  scale,  we 
must  not  consider  that  the  strata  have  always  assumed  their 
crystalline  or  altered  texture  in  consequence  of  the  proximity  of 
granite,  but  rather  that  granite  itself,  as  well  as  the  altered  strata, 
have  derived  their  crystalline  texture  from  plutonic  agency. 

In  accordance  with  this  hypothesis  I  have  proposed  (see 
Principles  of  Geology,)  the  term  "  Metamorphic"  for  the  altered 
strata,  a  term  derived  from  j«fa,  meta,  trans,  and  ftop^,  morphe, 
forma. 

Hence  there  are  four  great  classes  of  rocks  considered  in 
reference  to  their  origin, — the  aqueous,  volcanic,  plutonic,  and 
metamorphic,  all  of  which  may  be  conceived  to  have  been 
formed  contemporaneously  at  every  geological  period,  and  to  be 
now  in  the  progress  of  formation.  By  referring  to  the  Frontis- 
piece, the  reader  will  perceive  what  relative  positions  the  mem- 
bers of  these  four  great  classes  A,  B,  C,  D,  may  occupy  in  the 
earth's  crust,  while  in  the  course  of  simultaneous  production. 
Thus,  while  the  aqueous  deposits  A,  which  are  expressed  by  the 
yellow  colour,  have  been  accumulating  in  successive  strata  at  the 
bottom  of  the  sea,  the  volcanic  cone  B,  has  been  piled  up  during 
a  long  series  of  eruptions,  and  the  other  igneous  rocks  coloured 
purple  have  also  ascended  from  below  in  a  fluid  state.  Some  of 
these  last  have  been  poured  forth  into  the  sea,  and  there  mingled 
with  aqueous  sediment.  On  pursuing  downwards  either  the 
small  dikes  or  large  masses  of  volcanic  rock,  we  find  them  pass 
gradually  into  plutonic  formations,  D,  which  are  coloured  red, 
and  which  underlie  all  the  rest.  These  last  again  are  seen  to 
be  in  contact  with  a  zone  of  contemporaneous  metamorphic 
strata,  C,  coloured  blue,  which  they  penetrate  in  numerous  veins. 

In  that  part  of  the  section  which  is  uncoloured,  a  more  ancient 
series  of  mineral  masses  are  seen,  belonging  also  to  the  four 


PART  I.     CHAPTER  I.  23 

Contemporaneous  origin  of  Four  Classes  of  Rocks. 

great  divisions  of  rocks.  The  strata  from  a  to  i  represent  as 
many  distinct  aqueous  formations,  which  have  originated  at  dif- 
ferent periods,  and  are  each  distinguished  by  their  peculiar  fos- 
sils. The  mass  v  v  is  of  volcanic  origin,  and  was  formed  at  one 
of  those  periods,  namely,  when  the  strata  g  were  deposited.  The 
strata  m  m  are  ancient  metamorphic  formations,  and  the  rocks 
1,  2,  are  plutonic,  also  ancient,  but  of  different  dates. 

Now  it  will  be  shown  in  the  course  of  this  volume,  that  por- 
tions of  each  of  these  four  distinct  classes  of  rocks  have  origin- 
ated at  many  successive  periods.     It  is  not  true,  as  was  formerly 
supposed,  that  all  granite,  together  with  the  crystalline  or  meta- 
morphic strata,  were  first  formed,  and  therefore  entitled  to  be 
called  "  primitive,"  and  that  the  aqueous  and  volcanic  rocks  were 
afterwards  superimposed,  and  should,  therefore,  rank  as  second- 
ary in  the  order  of  time.     This  idea  was  adopted  in  the  infancy 
of  the  science,  when  all  formations,  whether  stratified  or  unstra- 
tified,  earthy  or  crystalline,  with  or  without  fossils,  were  alike 
regarded  as  of  aqueous  origin.     At  that  period  it  was  naturally 
argued,  that  the  foundation  must  be  older  than  the  superstructure. 
Granite,  as  being  the  lowest  rock,  must  have  been  first  "  precipi- 
tated from  the  waters  of  the  primeval  ocean  which  originally 
invested  the  globe,"  then  the  crystalline,  and  finally  the  fossilife- 
rous  strata,  together  with  other  associated  rocks,  were  deposited. 
But  when  the  doctrine  of  the  igneous  origin  of  granite  was 
generally  adopted,  the  terms  primitive  and  primary,  as  embra- 
cing the  plutonic  and  metamorphic  rocks,  should  at  once  have 
been  banished  from  the  nomenclature  of  geology.     For  after  it 
had  been  first  proved  that  granite  had  originated  at  many  differ- 
ent epochs,  some  antecedent,  others  subsequent  to  the  origin  of 
many  fossiliferous  strata,  it  was  also  demonstrated  that  strata 
which  had  once  contained  fossils,  had  become  metamorphic  at 
different  periods ;  in  other  words,  some  of  the  rocks  termed  pri- 
mary were  newer  than  others  which  were  called  secondary.     A 
question,  therefore,  has  arisen,  whether  the  lower  crystalline  por- 
tions of  the  earth's  crust,  partially  modified  as  they  have  been, 
and  renewed  from  time  to  time,  are  newer  or  older,  regarded  as 
a  whole,  than  the  sedimentary  and  volcanic  formations.     Have 
the  operations  of  decay  and  repair  been  most  active  above  or 
below  1     The  same  question  might  be  asked1  with  respect  to  the 
relative  antiquity  of  the  foundations  and  the  buildings  in  certain 
ancient  cities,  such  as  Venice  or  Amsterdam,  which  are  supported 
on  wooden  piles — whether  in  the  course  of  ages  have  the  wooden 
props,  or  the  buildings  of  brick,  stone,  and  marble  which  they 
support,  proved  the  most  durable  ?     Which  have  been  renewed 
most  frequently  ?  for  the  piles,  when  rotten,  can  be  removed  one 


24       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Contemporaneous  origin  of  Four  Classes  of  Rocks. 

after  the  other  without  injury  to  the  buildings  above.  In  like 
manner  the  materials  of  the  lower  part  of  the  earth's  crust  may 
pass  from  a  solid  to  a  fluid  state,  and  may  then  again  become 
consolidated  ;  or  sedimentary  strata  may  assume  a  new  and  meta- 
morphic  texture,  while  the  strata  above  continue  unchanged,  or 
retain  characters  by  which  their  claim  to  high  antiquity  may  be 
recognized.  During  such  subterranean  mutations,  the  earth- 
quake may  shatter  and  dislocate  the  incumbent  crust,  or  the 
ground  may  rise  or  sink  slowly  and  insensibly  throughout  wide 
areas  ;*  or  there  may  be  volcanic  eruptions  here  and  there ;  but 
the  great  mass  may  not  undergo  such  an  alteration  as  to  be  re- 
generated and  composed  of  new  rocks. 

As  all  the  crystalline  rocks  may,  in  some  respects,  be  viewed 
as  belonging  to  one  great  family,  whether  they  be  stratified  or 
un stratified,  it  will  often  be  convenient  to  speak  of  them  by  one 
common  name.  But  the  use  of  the  term  primary  would  imply 
a  manifest  contradiction,  for  reasons  which  the  student  will  now 
comprehend.  Ic  is  indispensable,  therefore,  to  find  a  new  name, 
one  which  must  not  be  of  chronological  import,  and  must  express, 
on  the  one  hand,  some  peculiarity  equally  attributable  to  granite 
and  gneiss  (to  the  plutonic  as  well  as  the  altered  rocks,)  and, 
on  the  other,  must  have  reference  to  characters  in  which  those 
rocks  differ,  both  from  the  volcanic  and  from  the  unaltered  sedi- 
mentary strata.  I  have  proposed  in  the  Principles  of  Geology 
the  term  "  hypogene"  for  this  purpose,  derived  from  vrto,  under, 
and  ywoptu,,  to  be  born  ;  a  word  implying  the  theory  that  granite, 
gneiss,  and  the  other  crystalline  formations  are  alike  nether- 
formed  rocks,  or  rocks  which  have  not  assumed  their  present 
form  and  structure  at  the  surface.  It  is  true  that  all  metamorphic 
strata  must  have  been  deposited  originally  at  the  surface,  or  on 
that  part  of  the  exterior  of  the  globe  which  is  covered  by  water ; 
but,  according  to  the  views  above  set  forth,  they  could  never  have 
acquired  their  crystalline  texture,  unless  they  had  been  modified 
by  plutonic  agency  under  pressure  in  the  depths  of  the  earth. 

From  what  has  now  been  said,  the  reader  will  understand  that 
the  four  great  classes  of  rocks  may  each  be  studied  under  two 
distinct  points  of  view :  first,  they  may  be  studied  simply  as 
mineral  masses  deriving  their  origin  from  particular  causes,  and 
having  a  certain  composition,  form,  and  position  in  the  earth's 
crust,  or  other  characters  both  positive  and  negative,  such  as  the 
presence  or  absence  of  organic  remains.  In  the  second  place, 
the  rocks  of  each  class  may  be  viewed  as  a  grand  chronological 
series  of  monuments,  attesting  a  succession  of  events  in  the  for- 
mer history  of  the  globe  and  its  living  inhabitants. 

*  See  chap.  5. 


PART  I.     CHAPTER  II.  25 

Division  of  the  Work Mineral  Composition  of  Stratified  Rocks. 

I  shall  accordingly  divide  this  work  into  two  parts,  in  refer- 
ence to  these  two  modes  of  considering  each  family  of  rocks.  In 
the  first  part,  the  characters  of  the  aqueous,  volcanic,  plutonic, 
and  metamorphic  rocks  will  be  described,  without  reference  to 
their  ages,  or  the  periods  when  they  were  formed.  In  the  second, 
their  different  ages  will  be  considered,  and  I  shall  endeavour  to 
explain  the  rules  according  to  which  the  chronology  of  rocks  in 
each  of  the  four  classes  may  be  determined. 


CHAPTER  II. 


AQUEOUS   HOCKS — THEIR    COMPOSITION  AND  FOEMS  OP  STRATIFICATION. 

Mineral  composition  of  strata — Arenaceous  rocks — Argillaceous — Calcareous — 
Gypsum — Forms  of  stratification — Original  horizontality — thinning  out — Diagonal 
arrangement — Ripple  mark. 

FIRST,  then,  in  pursuance  of  the  arrangement  explained  in 
the  last  chapter,  we  have  to  examine  the  aqueous  or  sedimentary 
rocks,  which  are,  for  the  most  part,  distinctly  stratified,  and  con- 
tain fossils.  We  are  to  consider  them  with  reference  to  their 
mineral  composition,  external  appearance,  position,  mode  of  ori- 
gin, and  other  characters  which  belong  to  them  as  aqueous 
formations,  without  reference  to  their  age,  or  the  various  geolo- 
gical periods  when  they  may  have  originated. 

I  have  already  given  an  outline  of  the  data  which  lead  to  the 
belief  that  the  stratified  and  fossiliferous  rocks  were  originally 
deposited  under  water ;  but,  before  entering  into  a  more  detailed 
investigation,  it  will  be  desirable  to  say  something  of  the  ordinary 
materials  of  which  such  strata  are  composed.  These  may  be 
said  to  belong  principally  to  three  divisions,  the  arenaceous,  the 
argillaceous,  and  the  calcareous,  which  are  formed  respectively 
of  sand,  clay,  and  carbonate  of  lime.  Of  these,  the  arenaceous, 
or  sandy  masses,  are  chiefly  made  up  of  siliceous  or  flinty  grains; 
the  argillaceous,  or  clayey,  of  a  mixture  of  siliceous  matter,  with 
a  certain  proportion,  about  a  fourth  in  weight,  of  aluminous 
earth;5  and,  lastly,  the  calcareous  rocks  or  limestones  consist  of 
carbonic  acid  and  lime. 

Arenaceous  or  siliceous  rocks. — To  speak  first  of  the  sandy 
division :  beds  of  loose  sand  are  frequently  met  with,  of  which 
the  grains  consist  entirely  of  silex,  which  term  comprehends  all 
purely  siliceous  minerals,  as  quartz  and  common  flint.  Quartz 
is  silex  in  its  purest  form  ;  flint  usually  contains  some  admixture 
of  alumine  and  oxide  of  iron.  The  siliceous  grains  in  sand  and 
c 


26       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Mineral  Composition  of  stratified  Kocks. 

sandstone  are  usually  rounded,  as  if  by  the  action  of  running 
water ;  but  they  sometimes,  though  .more  rarely,  consist  of  small 
crystals,  as  if  they  had  been  chemically  precipitated  from  a  fluid 
containing  silex  in  solution. 

Sandstone  is  an  aggregate  of  such  grains,  which  often  cohere 
together  without,  any  visible  cement,  but  more  commonly  are 
bound  together  by  a  slight  quantity  of  siliceous  or  calcareous 
matter,  or  by  iron  or  clay.  In  nature  there  is  every  intermediate 
gradation,  from  perfectly  loose  sand,  to  the  hardest  sandstone. 
In  micaceous  sandstones  mica  is  abundant ;  and  the  thin  silvery 
plates  into  which  that  mineral  divides,  are  arranged  into  layers 
parallel  to  the  planes  of  stratification,  giving  a  slaty  or  laminated 
texture  to  the  rock. 

When  sandstone  is  coarse-grained,  it  is  usually  called  grit. 
If  the  grains  are  rounded,  and  large  enough  to  be  called  pebbles, 
it  becomes  a  conglomerate,  or  pudding-stone,  which  may  consist 
of  pieces  of  one  or  of  many  different  kinds  of  rock.  A  conglom- 
erate, therefore,  is  simply  gravel  bound  together  by  a  cement. 

Argillaceous  rocfcs.-r— Clay,  strictly  speaking,  is  a  mixture  of 
silex  or  flint,  with  a  large  proportion,  usually  about  one-fourth, 
of  the  substance  called  alumine,  or  argil ;  but,  in  common  lan- 
guage, any  earth  which  possesses  sufficient  ductility,  when 
kneaded  up  with  water,  to  be  fashioned  like  paste  by  the  hand, 
or  by  the  potter's  lathe,  is  called  a  clay  ;  and  such  clays  vary 
greatly  in  their  composition,  and  are,  in  general,  nothing  more 
than  mud  derived  from  the  decomposition  or  wearing  down  of 
various  rocks.  The  purest  clay  found  in  nature  is  porcelain 
clay,  or  kaolin,  which  results  from  the  decomposition  of  a  rock 
composed  of  felspar  and  quartz,  and  is  almost  always  mixed 
with  quartz.*  Shale  has  also  the  property,  like  clay,  of  be- 
coming plastic  in  water :  it  is  a  more  solid  form  of  clay,  having 
been  probably  condensed  by  pressure.  It  usually  divides  into 
thin  laminse. 

One  general  character  of  all  argillaceous  rocks  is  to  give  out 
a  peculiar  odour  when  breathed  upon,  which  is  a  test  of  the  pre- 
sence of  alumine,  although  it  does  not  belong  to  pure  alumine, 
but,  apparently,  to  the  combination  of  that  substance  with  oxide 
of  iron.f 

Calcareous  rocks. — This  division  comprehends  those  rocks 
which,  like  chalk,  are  composed  of  lime  and  carbonic  acid. 

*  The  kaolin  of  China  consists  of  71.15  parts  of  silex,  15.86  of  alumine,  1.92  of 
lime,  and  6.73  of  water,  (W.  Phillips,  Mineralogy,  p.  33.) ;  but  other  porcelain 
clays  differ  materially,  that  of  Cornwall  being  composed  of  CO  parts  of  alumine 
and  40  of  silex. 

t  See  W.  Phillips's  Mineralogy,  "  Alumine." 


Shells  and  corals  are  also  formed  of  the  sanW5ii^eiits1.with  ifie 
addition  of  animal  matter.  To  obtain  pure  lime,  it  is  necessary 
to  calcine  these  calcareous  substances,  that  is  to  say,  to  expose 
them  to  heat  of  sufficient  intensity  to  drive  off  the  carbonic  acid, 
and  other  volatile  matter,  without  vitrifying  or  melting  the  lime 
itself.  White  chalk  is  often  pure  carbonate  of  lime ;  and  this 
rock,  although  usually  in  a  soft  and  earthy  state,  is  sometimes 
sufficiently  solid  to  be  used  for  building,  and  even  passes  into  a 
compact  stone,  or  a  stone  of  which  the  separate  parts  are  so 
minute  as  not  to  be  distinguishable  from  each  other  by  the  naked 
eye. 

Many  limestones  are  made  up  entirely  of  minute  fragments  of 
shells  and  coral,  or  of  calcareous  sand  cemented  together.  These 
last  might  be  called  "  calcareous  sandstones  ;"  but  that  term  is 
more  properly  applied  to  a  rock  in  which  the  grains  are  partly 
calcareous  and  partly  siliceous,  or  to  quartzose  sandstones,  having 
a  cement  of  carbonate  of  lime. 

The  variety  of  limestone  called  "  oolite"  is  composed  of  numer- 
ous small  egg-like  grains,  resembling  the  roe  of  a  fish,  each  of 
which  has  usually  a  sma*ll  fragment  of  sand  as  nucleus,  around 
which  concentric  layers  of  calcareous  matter  have  accumulated. 

Any  limestone  which  is  sufficiently  hard  to  take  a  fine  polish 
is  called  marble.  Many  of  these  are  fossiliferous  ;  but  statuary 
marble,  which  is  also  called  saccharine  limestone,  as  having  a 
texture  resembling  that  of  loaf-sugar,  is  devoid  of  fossils,  and  a 
member  of  the  metamorphic  series. 

Siliceous  limestone  is  an  intimate  mixture  of  carbonate  of 
lime  and  flint,  and  is  harder  in  proportion  as  the  flinty  matter 
predominates. 

The  presence  of  carbonate  of  lime  in  a  rock  may  be  ascer- 
tained by  applying  to  the  surface  a  small  drop  of  diluted  sul- 
phuric, nitric,  or  muriatic  acids  ;  for  the  lime,  having  a  stronger 
chemical  affinity  for  any  one  of  these  acids  than  for  the  carbonic, 
unites  itself  immediately  with  them  to  form  new  .compounds, 
thereby  becoming  a  sulphate,  nitrate,  or  muriate  of  lime.  The 
carbonic  acid,  when  thus  liberated  from  its  union  with  the  lime, 
escapes  in  a  gaseous  form,  and  froths  up  or  effervesces  as  it 
makes  its  way  in  small  bubbles  through  the  drop  of  liquid.  This 
effervescence  is  brisk  or  feeble  in  proportion  as  the  limestone  is 
pure  or  impure,  or,  in  other  words,  according  to  the  quantity  of 
foreign  matter  mixed  with  the  carbonate  of  lime.  Without  the 
aid  of  this  test,  the  most  experienced  eye  cannot  always  detect 
the  presence  of  lime  in  rocks. 

The  above-mentioned  three  classes  of  rocks,  the  arenaceous, 
argillaceous,  and  calcareous,  pass  continually  into  each  other, 


28       LYELL'S  ELEMENTS  OF  GEOLOGY. 

-      Forms  of  Stratification. 

and  rarely  occur  in  a  perfectly  separate  and  pure  form.  Thus 
it  is  an  exception  to  the  general  rule  to  meet  with  a  limestone  as 
pure  as  ordinary  white  chalk,  or  with  clay  as  aluminous  as  that 
used  in  Cornwall  for  porcelain,  or  with  sand  so  entirely  com- 
posed of  siliceous  grains  as  the  white  sand  of  Alum  Bay  in  the 
Isle  of  Wight,  or  sandstone  so  pure  as  the  grit  of  Fontainebleau, 
used  for  pavement  in  France.  More  commonly  \ve  find  sand 
and  clay,  or  clay  and  marl,  intermixed  in  the  same  mass.  When 
the  sand  and  clay  are  each  in  considerable  quantity,  the  mixture 
is  called  loam.  If  there  is  much  calcareous  matter  in  clay,  it  is 
called  marl;  but  this  term  has  unfortunately  been  used  so 
vaguely,  as  often  to  be  very  ambiguous.  It  has  been  applied  to 
substances  in  which  there  is  no  lime ;  as,  to  that  red  loarmisually 
called  red  marl  in  certain  parts  of  England.  Agriculturists  were 
in  the  habit  of  calling  any  soil  a  marl,  which,  like  true  marl, 
fell  to  pieces  readily  on  exposure  to  the  air.  Hence  arose  the 
confusion  of  using  this  name  for  soils  which,  consisting  of  loam, 
were  easily  worked  by  the  plough,  though  devoid  of  lime. 

Marl  slate  bears  the  same  relation  to  marl  which  shale  bears 
to  clay,  being  a  calcareous  shale.  It  is  very  abundant  in  some 
countries,  as  in  the  Swiss  Alps.  Argillaceous  or  marly  lime- 
stone is  also  of  common  occurrence. 

There  are  few  other  kinds  of  rock  which  enter  so  largely  into 
the  composition  of  sedimentary  strata  as  to  make  it  necessary  to 
dwell  here  on  their  characters.  I  may,  however,  mention  two 
others, — magnesian  limestone  or  dolomite,  and  gypsum.  Mag- 
nesian  limestone  is  composed  of  carbonate  of  lime  and  carbo- 
nate of  magnesia :  the  proportion  of  the  latter  amounting  in 
some  cases  to  nearly  one  half.  It  effervesces  much  more  slowly 
and  feebly  with  acids  than  common  limestone.  In  England  this 
rock  is  generally  of  a  yellowish  colour ;  but  it  varies  greatly  in 
mineralogical  character,  passing  from  an  earthy  state  to  a  white 
compact  stone  of  great  hardness.  Dolomite,  so  common  in  many 
parts  of  Germany  and  France,  is  also  a  variety  of  magnesian 
limestone,  usually  of  a  granular  texture. 

Gypsum.  —  Gypsum  is  a  rock  composed  of  sulphuric  acid, 
lime,  and  water.  It  is  usually  a  soft  whitish-yellow  rock,  with 
a  texture  resembling  that  of  loaf-sugar,  but  sometimes  it  is 
entirely  composed  of  lenticular  crystals.  It  is  insoluble  in  acids, 
and  does  not  effervesce  like  chalk  and  dolomite,  the  lime  being 
already  combined  with  sulphuric  acid,  for  which  it  has  a  stronger 
affinity  than  for  any  other.  Anhydrous  gypsum  is  a  rare  variety, 
into  which  water  does  not  enter  as  a  component  part.  Gypseous 
marl  is  a  mixture  of  gypsum  and  marl. 

Forms  of  stratification. — A  series  of  strata  sometimes  con- 


PART  I.     CHAPTER  II.  29 

Alternations Sorting  Power  of  Water. 

sists  of  one  of  the  above  rocks,  sometimes  of  two  or  more  in 
alternating  beds.  Thus,  in  the  coal  districts  of  England,  for 
example,  we  often  pass  through  several  beds  of  sandstone,  some 
of  finer,  others  of  coarser  grain,  some  white,  others  of  a  dark 
colour,  and  below  these,  layers  of  shale  and  sandstone  or  beds 
of  shale,  divisible  into  leaf-like  laminse,  and  containing  beautiful 
impressions  of  plants.  Then  again  we  meet  with  beds  of  pure 
and  impure  coal,  alternating  with  shales,  and  underneath  the 
whole,  perhaps,  are  calcareous  strata,  or  beds  of  limestone, 
filled  with  corals  and  marine  shells,  each  bed  distinguishable 
from  another  by  certain  fossils,  or  by  the  abundance  of  particu- 
lar species  of  shells  or  zoophytes. 

This  alternation  of  different  kinds  of  rock  produces  the  most 
distinct  stratification ;  and  we  often  find  beds  of  limestone  and 
marl,  conglomerate  and  sandstone,  sand  and  clay,  recurring 
again  and  again,  in  nearly  regular  order,  throughout  a  series  of 
many  hundred  strata.  The  causes  which  may  produce  these 
phenomena  are  various,  and  have  been  fully  discussed  in  my 
treatise  on  the  modern  changes  of  the  earth's  surface.*  It  is 
there  seen  that  rivers  flowing  into  lakes  and  seas  are  charged 
with  sediment,  varying  in  quantity,  composition,  colour,  and 
grain,  according  to  the  seasons  ;  the  waters  are  sometimes  flooded 
and  rapid,  at  other  periods  low  and  feeble ;  different  tributaries, 
also,  draining  peculiar  countries  and  soils,  and  therefore  charged 
with  peculiar  sediment,  are  swollen  at  distinct  periods.  It  was 
also  shown  that  the  waves  of  the  sea  and  currents  undermine  the 
cliffs  during  wintry  storms,  and  sweep  away  the  materials  into 
the  deep,  after  which  a  season  of  tranquillity  succeeds,  when 
nothing  but  the  finest  mud  is  spread  by  the  movements  of  the 
ocean  over  the  same  submarine  area. 

It  is  not  the  object  of  the  present  work  to  give  a  description  of 
these  operations,  repeated  as  they  are,  year  after  year,  and  cen- 
tury after  century;  but  I  may  suggest  an  explanation  of  the 
manner  in  which  some  micaceous  sandstones  have  originated, 
those  in  which  we  see  innumerable  thin  layers  of  mica  dividing 
layers  of  fine  quartzose  sand.  I  observed  the  same  arrangement 
of  materials  in  recent  mud  deposited  in  the  estuary  of  La  Roche 
St.  Bernard  in  Brittany,  at  the  mouth  of  the  Loire.  The  sur- 
rounding rocks  are  of  gneiss,  which,  by  its  waste,  supplies  the 
mud  :  when  this  dries  at  low  water,  it  is  found  to  consist  of  brown 
laminated  clay,  divided  by  thin  seams  of  mica.  The  separation 
of  the  mica,  in  this  case,  or  in  that  of  micaceous  sandstones,  may 

*  Consult  Index  to  Principles  of  Geology.     "  Stratification,"    "  Currents," 
«  Deltas,"  «  Water,"  &c. 


30       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Horizontality  of  Strata. 

be  thus  understood.  If  we  take  a  handful  of  quartzose  sand, 
mixed  with  mica,  and  throw  it  into  a  clear  running  stream,  we 
see  the  materials  immediately  sorted  by  the  water,  the  grains  of 
quartz  falling  almost  directly  to  the  bottom,  while  the  plates  of 
mica  take  a  much  longer  time  to  reach  the  bottom,  and  are  car- 
ried farther  down  the  stream.  At  the  first  instant  the  water  is 
turbid,  but  immediately  after  the  flat  surfaces  of  the  plates  of 
mica  are  seen  alone  reflecting  a  silvery  light,  and  they  descend 
slowly,  to  form  a  distinct  micaceous  lamina.  The  mica  is  the 
heavier  mineral  of  the  two ;  but  it  remains  longer  suspended, 
owing  to  its  great  extent  of  surface.  It  is  easy,  therefore,  to 
conceive  how  the  intermittent  action  of  waves,  currents,  and 
tides,  may  sort  the  sediment  brought  down  from  the  waste  of  a 
granitic  country,  and  throw  down  the  mica,  layer  after  layer, 
separately  from  the  mud  or  sand. 

Original  hcrizontality. — It  has  generally  been  said  that  the 
upper  and  under  surfaces  of  strata,  or  the  planes  of  stratification, 
as  they  are  termed,  are  parallel.  Although  this  is  not  strictly 
true,  they  make  an  approach  to  parallelism,  for  the  same  reason 
that  sediment  is  usually  deposited  at  first  in  nearly  horizontal 
layers.  The  reason  of  this  arrangement  can  by  no  means  be 
attributed  to  an  original  evenness  or  horizontally  in  the  bed  of 
the  sea ;  for  it  is  ascertained  that  in  those  places  where  no  matter 
has  been  recently  deposited,  the  bottom  of  the  ocean  is  often  as 
uneven  as  that  of  the  dry  land,  having,  in  like  manner,  its  hills, 
valleys,  and  ravines.  Yet  if  the  sea  should  sink,  or  the  water 
be  removed  near  the  mouth  of  a  large  river  where  a  delta  has 
been  forming,  we  should  see  extensive  plains  of  mud  and  sand 
laid  dry,  which,  to  the  eye,  would  appear  perfectly  level,  al- 
though, in  reality,  they  would  slope  gently  from  the  land  towards 
the  sea.  This  tendency  in  newly-formed  strata  to  assume  a 
horizontal  position,  arises  principally  from  the  motion  of  the 
water,  which  forces  along  particles  of  sand  or  mud  at  the  bottom, 
and  causes  them  to  settle  in  hollows  or  depressions,  where  they 
are  less  exposed  to  the  force  of  a  current  than  when  they  are 
resting  on  elevated  points.  The  velocity  of  the  current  and  the 
motion  of  the  superficial  waves  diminish  from  the  surface  down- 
wards, and  is  least  in  those  depressions  where  the  water  is  deep- 
est. A  good  illustration  of  the  principle  here  alluded  to,  may  be 
sometimes  seen  in  the  neighbourhood  of  a  volcano,  when  a  sec- 
tion, whether  natural  or  artificial,  has  laid  open  to  view  a  suc- 
cession of  various-coloured  layers  of  sand  and  ashes,  which  have 
fallen  in  showers  upon  uneven  ground.  Thus,  let  A,  B  (Fig.  1.) 
be  two  ridges,  with  an  intervening  valley.  These  original  in- 
equalities of  the  surface  have  been  gradually  effaced  by  beds  of 


PART  I.     CHAPTER  II. 


81 


Original  Horizon  tali  ty Thinning  out. 

sand  and  ashes  c  d  e,  the  surface  at  e  being  quite  level.  It  will 
be  seen,  that  although  the  materials  of  the  first  layers  have  ac- 
commodated themselves,  in  a  great  degree,  to  the  shape  of  the 
ground  A  B,  yet  each  bed  is  thickest  at  the  bottom.  At  first,  a 
j?jp.  j  great  many  particles  would  be  car- 

ried by  their  own  gravity  down  the 
steep  sides  of  A  and  B,  and  others 
would  afterwards  be  blown  by  the 
wind  as  they  fell  off  the  ridges,  and 


would  settle  in  the  hollow,  which  would  thus  become  more  and 
more  effaced  as  the  strata  accumulated  from  c  to  e.  This  level- 
ling operation  may,  perhaps,  be  rendered  more  clear  to  the  stu- 
dent, by  supposing  a  number  of  parallel  trenches  to  be  dug  in  a 
plain  of  moving  sand,  like  the  African  desert,  in  which  case  the 
wind  would  soon  cause  all  the  signs  of  these  trenches  to  disap- 


pea 


and  the  surface  would  be  as  uniform  as  before.  Now,  water 


in  motion  can  exert  this  levelling  power  on  similar  materials 
more  easily  than  air,  for  almost  all  stones  lose  in  water  more 
than  a  third  of  the  weight  which  they  have  in  air,  the  specific 
gravity  of  rocks  being  in  general  as  2^  when  compared  to  that 
of  water,  which  is  estimated  at  1.  But  the  buoyancy  of  sand 
or  mud  would  be  still  greater  in  the  sea,  as  the  density  of  salt 
water  exceeds  that  of  fresh. 

Yet,  however  uniform  and  horizontal  may  be  the  surface  of 
new  deposits  in  general,  there  are  still  many  disturbing  causes, 
such  as  eddies  in  the  water,  and  currents  moving  first  in  one 
and  then  in  another  direction,  which  frequently  cause  irregulari- 
ties. We  may  sometimes  follow  a  bed  of  limestone,  shale,  or 
sandstone,  for  a  distance  of  many  hundred  yards  continuously ; 
but  we  generally  find  at  length  that  each  individual  stratum  thins 
Fig.  2. 


Section  of  strata  of  sandstone,  grit,  and  conglomerate. 

out,  and  allows  the  beds  which  were  previously  above  and  below 
it  to  meet.  If  the  materials  are  C^MMC,  as  in  grits  and  con- 
glomerates, the  same  beds  can  rarely  be  traced  many  yards 
without  varying  in  size,  and  often  coming  to  an  end  abruptly. 
(See  Fig.  2.) 

There  is  also  another  phenomenon  of  frequent  occurrence. 
We  find  a  series  of  larger  strata,  each  of  which  is  composed  of 
a  number  of  minor  layers  placed  obliquely  to  the  general  planes 
of  stratification.  To  this  diagonal  arrangement  the  name  of 


32 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Diagonal  Stratification,  and  its  Causes. 


"  false  stratification"  has  been  given.    Thus  in  the  annexed  sec- 
tion (Fig.  3.)  we  see  seven  or  eight  large  beds  of  loose  sand, 


Fig.  3. 


Section  of  sand  at  Sandy  Hall,  near  JBiggleswade,  Bedfordshire, 
Height  twenty  feet.    (Green-sand  formation.) 

yellow  and  brown,  and  the  lines  a,  &,  c,  mark  some  of  the  prin- 
cipal planes  of  stratification,  which  are  nearly  horizontal.  But 
the  greater  part  of  the  subordinate  laminae  do  not  conform  to 
these  planes,  but  have  often  a  steep  slope,  the  inclination  being 
sometimes  towards  opposite  points  of  the  compass.  When  the 
sand  is  loose  and  incoherent,  as  in  the  case  here  represented,  the 
devia^n  from  parallelism  of  the  slanting  lamina?  cannot  possi- 
bly be  accounted  for  by  any  rearrangement  of  the  particles 
acquired  during  the  consolidation  of  the  rock.  In  what  manner 
then  can  such  irregularities  be  due  to  original  deposition  ?  We 
must  suppose  that  at  the  bottom  of  the  sea,  as  well  as  in  the  beds 
of  rivers,  the  motions  of  waves,  currents,  and  eddies  often  cause 
mud,  sand,  and  gravel  to  be  thrown  down  in  heaps  on  particular 
spots,  instead  of  being  spread  out  uniformly  over  a  wide  area. 
Sometimes,  when  banks  are  thus  formed,  currents  may  cut  pas- 
sages through  them,  just  as  a  river  forms  its  bed.  Suppose  the 
bank  A  (Fig.  4.)  to  be  thus  formed  with  a  steep  sloping  side,  and 
Fig.  4.  B 


mmm 


C  D 

the  water  being  in  a  tranquil  state,  the  layer  of  sediment  No.  1- 


PART  I.     CHAPTER  II. 


33 


Ripple  Mark,  how  formed. 


is  thrown  down  upon  it,  conforming  nearly  to  its  surface.  After- 
wards the  other  layers,  2,  3,  4,  may  be  deposited  in  succession, 
so  that  the  bank  B  C  D  is  formed.  If  the  current-then  increases 
in  velocity,  it  may  cut  away  the  upper  portion  of  this  mass  down 
to  the  dotted  line  e  (Fig.  4.),  and  deposit  the  materials  thus 
removed  farther  on,  so  as  to  form  the  layers  5,  6, 7,  8.  We  have 
now  the  bank  B  C  D  E  (Fig.  5.,)  of  which  the  surface  is  almost 

Fig.  5. 


level,  and  on  which  the  nearly  horizontal  layers  9,  10,  11,  may 
then  accumulate.  The  opposite  slope  of  the  diagonal  layers  of 
successive  strata,  in  the  section  Fig.  3.,  may  be  accounted  for  by 
changes  in  the  direction  of  the  tides  and  currents  in  the  same 
place. 

Fig.  6. 


Slab  of  ripple-marked  (new  red}  sandstone  from  Cheshire. 

The  ripple  mark,  so  common  on  the  surface  of  sandstones  of 
all  ages  (see  Fig.  6.)  and  which  is  so  often  seen  on  the  sea-shore 


34       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Ripple  Mark,  how  formed. 

at  low  tide,  seems  to  originate  in  the  drifting  of  materials  along 
the  bottom  of  the  water,  in  a  manner  very  similar  to  that  which 
may  explain  the  inclined  layers  above  described.  This  ripple  is 
not  entirely  confined  to  the  beach  between  high  and  low  water 
mark,  but  is  also  produced  on  sands  which  are  constantly  covered 
by  water.  Similar  undulating  ridges  and  furrows  may  also  be 
sometimes  seen  on  the  surface  of  drift  snow  and  blown  sand. 
The  following  is  the  manner  in  which  I  once  observed  the  motion 
of  the  air  to  produce  this  effect  on  a  large  extent  of  level  beach, 
exposed  at  low  tide  near  Calais.  Clouds  of  fine  white  sand 
were  blown  from  the  neighbouring  dunes,  so  as  to  cover  the 
shore,  and  whiten  a  dark  level  surface  of  sandy  mud,  and -this 
fresh  covering  of  sand  was  beautifully  rippled.  On  levelling  all 
the  small  ridges  and  furrows  of  this  ripple  over  an  area  several 
yards  square,  I  saw  them  perfectly  restored  in  about  ten  minutes, 
the  general  direction  of  the  ridges  being  always  at  right  angles 
to  that  of  the  wind.  The  restoration  began  by  the  appearance 
here  and  there  of  small  detached  heaps  of  sand,  which  soon 
lengthened  and  joined  together,  so  as  to  form  long  sinuous  ridges 
with  intervening  furrows.  Each  ridge  had  one  side  slightly 
Fig.  7. 


inclined,  and  the  other  steep ;  the  lee  side  being  always  steep, 
as  b,  c, — d,  e;  the  windward  side  a  gentle  slope,  as  a,  &,  —  c,  dt 
Fig.  7.  When  a  gust  of  wind  blew  with  sufficient  force  to  drive 
along  a  cloud  of  sand,  all  the  ridges  were  seen  to  be  in  motion 
at  once,  each  encroaching  on  the  furrow  before  it,  and,  in  the 
course  of  a  few  minutes,  filling  the  place  which  the  furrows  had 
occupied.  The  mode  of  advance  was  by  the  continual  drifting 
of  grains  of  sand  up  the  slopes  a  b  and  c  d,  many  of  which 
grains,  when  they  arrived  at  b  and  d,  fell  over  the  scarps  b  c  and 
d  e,  and  were  under  shelter  from  the  wind  ;  so  that  they  remained 
stationary,  resting,  according  to  their  shape  and  momentum,  on 
different  parts  of  the  descent,  and  a  few  only  rolling  to  the  bot- 
tom. In  this  manner  each  ridge  was  distinctly  seen  to  move 
slowly  on  as  often  as  the  force  of  the  wind  augmented.  Occa- 
sionally part  of  a  ridge,  advancing  more  rapidly  than  the  rest, 
overtook  the  ridge  immediately  before  it,  and  became  confounded 
with  it,  thus  causing  those  bifurcations  and  branches  which  are 
so  common,  and  two  of  which  are  seen  in  the  slab  Fig.  6.  We 
may  observe  this  configuration  in  sandstones  of  all  ages,  and  in 
them  also,  as  now  on  the  sea-coast,  we  may  often  detect  two  sys- 


PART  I.     CHAPTER  III.  35 


Arrangement  of  Fossils  in  Strata. 


terns  of  ripples  interfering  with  each  other ;  one  more  ancient 
and  half  effaced,  and  a  newer  one,  in  which  the  grooves  and 
ridges  are  more  distinct,  and  in  a  different  direction.  This  cross- 
ing of  two  sets  of  ripples  arises  from  a  change  of  wind,  and  the 
new  direction  in  which  the  waves  are  thrown  on  the  shore. 


CHAPTER  III. 


ARRANGEMENT  OF  FOSSILS  IN  STRATA — FRESHWATER  AND  MARINE. 

Successive  deposition  indicated  by  fossils  —  Limestones  formed  of  corals  and 
shells — Proofs  of  gradual  increase  of  strata  derived  from  fossils — Serpula 
attached  tospatangus  —  Wood  bored  byteredina  —  Tripoli  and  semi-opal  formed 
of  infusoria  —  Chalk  derived  principally  from  organic  bodies  —  Distinction  of 
freshwater  from  marine  formations — Genera  of  freshwater  and  land  shells  — 
Rules  for  recognizing  marine  testacea  —  Gyrogonite  and  chara  —  Freshwater 
fishes  —  Alternation  of  marine  and  freshwater  deposits  —  Lym-Fiord. 

HAVING  in  the  last  chapter  considered  the  forms  of  stratifica- 
tion so  far  as  they  are  determined  by  the  arrangement  of  inor- 
ganic matter,  we  may  now  turn  our  attention  to  the  manner  in 
which  organic  remains  are  distributed  through  stratified  deposits. 
We  should  often  be  unable  to  detect  any  signs  of  stratification  or 
of  successive  deposition,  if  particular  kinds  of  fossils  did  not 
occur  here  and  there  at  certain  depths  in  the  mass.  At  one  level, 
for  example,  bivalve  shells  of  some  one  or  more  species  pre- 
dominate ;  at  another,  some  univalve  shell,  and  at  a  third,  corals  ; 
while  in  some  formations  we  find  layers  of  vegetable  matter 
separating  strata. 

It  may  appear  inconceivable  to  a  beginner  how  mountains, 
several  thousand  feet  thick,  can  have  become  filled  with  fossils 
from  top  to  bottom  ;  but  the  difficulty  is  removed  when  he  reflects 
on  the  origin  of  stratification,  as  explained  in  the  last  chapter, 
and  allows  sufficient  time  for  the  accumulation  of  sediment.  He 
must  never  lose  sight  of  the  fact  that,  during  the  process  of  depo- 
sition, each  separate  layer  was  once  the  uppermost,  and  covered 
immediately  by  the  water  in  which  aquatic  animals  lived.  Each 
stratum,  in  fact,  however  far  it  may  now  lie  beneath  the  surface, 
was  once  in  the  state  of  loose  sand  or  soft  mud  at  the  bottom  of 
the  sea,  in  which  shells  and  other  bodies  easily  became  enveloped. 

By  attending  to  the  nature  of  these  remains,  we  are  often 
enabled  to  determine  whether  the  deposition  was  slow  or  rapid, 
whether  it  took  place  in  a  deep  or  shallow  sea,  near  the  shore  or 
far  from  land,  and  whether  the  water  was  salt,  brackish,  or  fresh. 


36 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Gradual  Deposition  indicated  by  Fossils. 


Some  limestones  consist  almost  exclusively  of  corals,  and  their 
position  has  evidently  been  determined  by  the  manner  in  which 
the  zoophytes  grew ;  for  if  the  stratum  be  horizontal,  the  round 
spherical  head  of  certain  species  is  uppermost,  and  the  point  of 
attachment  directed  downwards.  This  arrangement  is  sometimes 
repeated  throughout  a  great  succession  of  strata.  From  what 
we  know  of  the  growth  of  similar  zoophytes  in  modern  reefs,  we 
infer  that  the  rate  of  increase  was  extremely  slow,  and  some  of 
the  fossils  must  have  flourished  for  ages  like  forest  trees,  before 
they  attained  so  large  a  size.  During  these  ages,  the  water  re- 
mained clear  and  transparent,  for  such  zoophytes  cannot  live  in 
turbid  water. 

In  like  manner,  when  we  see  thousands  of  full-grown  shells 
dispersed  every  where  throughout  a  long  series  of  strata,  we  can- 
not doubt  that  time  was  required  for  the  multiplication  of  suc- 
cessive generations ;  and  the  evidence  of  slow  accumulation  is 
rendered  more  striking  from  the  proofs,  so  often  discovered,  of 
fossil  bodies  having  lain  for  a  time  on  the  floor  of  the  ocean  after 
death,  before  they  were  imbedded  in  sediment.  Nothing,  for 
example,  is  more  common  than  to  see  fossil  oysters  in  clay,  with 
serpulae,  acorn-shells,  corals,  and  other  creatures,  attached  to  the 
inside  of  the  valves,  so  that  the  mollusk  was  certainly  not  buried 
Fig.  8.  in  argillaceous  mud  the 

moment  it  died.  There 
must  have  been  an  interval 
during  which  it  was  still 
surrounded  with  clear  wa- 
ter, when  the  testacea,  now 
adhering  to  it,  grew  from 
an  embryo  state  to  full  ma- 
turity. Attached  shells, 
which  are  merely  external, 
like  some  of  the  serpulse 
in  the  annexed  figure,  (Fig. 
8.)  may  often  have  grown 
upon  an  oyster,  or  other 
shell,  while  the  animal  with- 
in was  still  living ;  but  if 
they  are  found  on  the  in- 
side, it  could  only  happen 
after  the  death  of  the  in- 
habitant of  the  shell  which 
affords  the  support.  Thus, 
in  Fig.  8.,  it  will  be  seen 


Fossil  Gryjifuza,  covered  both  on  the  outside  and 
inside  with  fossil  serpula;. 


PART  I.     CHAPTER  III. 


37 


Gradual  Deposition  indicated  by  Fossils. 


Fig.  9. 


.  10. 


that  two  serpulse  have  grown  on  the  interior,  one  of  them  ex- 
actly on  the  place  where  the  adductor  muscle  of  the  Gryphtra 
(a  kind  of  oyster)  was  fixed. 

Some  fossil  shells,  even  if  simply  attached  to  the  outside  of 
others,  bear  full  testimony  to  the  conclusion  above  alluded  to, 
namely,  that  an  interval  elapsed  between  the  death  of  the  crea- 
ture to  whose  shell  they  adhere,  and  the  burial  of  the  same  in 
mud  or  sand.  The  sea-urchins,  or  Echini,  so  abundant  in  white 
chalk,  afford  a  good  illustration.  It  is  well  known  that  these 
animals,  when  living,  are  invariably  covered  with  numerous 
spines,  which  serve  as  organs  of  motion,  and  are  supported  by 
rows  of  tubercles,  which  last  are  only  seen  after  the  death  of  the 
sea-urchin,  when  the  spines  have  dropped  off.  In  Fig.  10.  a 

living  species  of  Spa- 
tangus,  common  on 
our  coast,  is  repre- 
sented with  one-half 
of  its  shell  stripped 
of  the  spines.  In  Fig. 
9,  a  fossil  of  the  same 
genus  from  the  white 

Recent  Spatangus7witk  the  chalk     of      England 
spines  removed  from  one  shows  the  naked  Slir- 

&.tp!ne    and    tubercles,  f*<*  which  the >  indi- 
natural  size.  Vlduals  of  this  family, 

a.  The  same  magnified,  exhibit  when  denuded 
of  their  bristles.  The  full-grown  Serpula,  therefore,  which  now 
adheres  externally,  could  not  have  begun  to  grow  till  the  Spa- 
tangus  had  died,  and  the  spines  were  detached. 

Now,  the  series  of  events  here  attested  by  a  single  fossil  may 
be  carried  a  step  farther.  Thus,  for  example,  we  often  meet 
with  a  sea-urchin  in  the  chalk,  (see  Fig.  11.),  which  has  fixed  to 
it  the  lower  valve  of  a  crania,  an  extinct  genus 
of  bivalve  mollusca.  The  upper  valve  (6  Fig. 
11.)  is  almost  invariably  wanting,  though  occa- 
sionally found  in  a  perfect  state  of  preservation 
in  white  chalk  at  some  distance.  In  this  case, 
we  see  clearly  that  the  sea-urchin  first  lived 
from  youth  to  age,  then  died  and  lost  its  spines 
which  were  carried  away.  Then  the  young 
a.  Echinus  from  the  Crania  adhered  to  the  bared  shell,  and  perish- 

va?ie'  onSJ ^ranTa  ed  in  its  turn  ?  after  which  the  UPPer  Valve  Was 

attached.  separated  from  the  lower  before  the  Echinus 

'  cSia  detached. the  became  enveloped  in  chalky  mud. 
It  may  be  well  to  mention  one  more  illustration  of  the  manner 
D 


Serpula  attached  to 
fossil  Spalangus 
from  the  chalk. 


38 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Gradual  Deposition  indicated  by  Fossils. 


in  which  single  fossils  may  sometimes  throw  light  on  a  former 
state  of  things,  both  in  the  bed  of  the  ocean  and  on  some  ad- 
joining land.  We  meet  with  many  fragments  of  wood  bored  by 
ship-worms  ac  various  depths  in  the  clay  on  which  London  is 
built.  Entire  branches  and  stems  of  trees,  several  feet  in  length, 
are  sometimes  dug  out,  drilled  all  over  by  the  holes  of  these 
borers,  the  tubes  and  shells  of  the  mollusk  still  remaining  in  the 
cylindrical  hollows.  In  Fig.  13.,  e,  a  representation  is  given  of 
a  piece  of  recent  wood  pierced  by  the  Teredo  navalis,  or  com- 
mon ship-worm,  which  destroys  wooden  piles  and  ships.  When 

Fig.  12. 


Fig.  13. 


Fossil  and  recent  wood  drilled  by  perforating  mollusca. 

Fig.  12.  a.  Fossil  wood  from  London  clay,  bored  by  Teredina. 

b.  Shell  and  tube  of  Teredina  personata,  the  right  hand  figure  the  ven- 

tral, the  left  the  dorsal  view. 
Fig.  13.  e.  Recent  wood  bored  by  Teredo. 

d.  Shell  and  tube  of  Teredo  navolis,  from  the  same.  • 

c.  Anterior  and  posterior  view  of  the  valves  of  the  same  detached  from 

the  tube. 

the  cylindrical  tube  d  has  been  extracted  from  the  wood,  a  shell 
is  seen  at  the  larger  extremity,  composed  of  two  pieces,  as  shown 
at  c.  In  like  manner,  a  piece  of  fossil  wood  (a,  Fig.  12.)  has 
been  perforated  by  an  animal  of  a  kindred  but  extinct  genus 
called  Teredina  by  Lamarck.  The  calcareous  tube  of  this  mol- 
lusk was  united,  and,  as  it  were,  soldered  on  to  the  valves  of  the 
shell,  (b)  which  therefore  cannot  be  detached  from  the  tube,  like 
the  valves  of  the  recent  Teredo.  The  wood  in  this  fossil  sped- 


PART  I.  CHAPTER  III. 


Infusoria  in  Tripoli. 


men  is  now  converted  into  a  stony  mass,  a  mixture  of  clay 
and  lime ;  but  it  must  once  have  been  buoyant  and  floating  in 
the  sea,  when  the  TeredinaB  lived  upon  it,  perforating  it  in  all 
directions.  Again,  before  the  infant  colony  settled  upon  the  drift 
wood,  the  branch  of  a  tree  must  have  been  floated  down  to  the 
sea  by  a  river,  uprooted,  perhaps,  by  a  flood,  or  torn  offatid  cast 
into  the  waves  by  wind ;  and  thus  our  thoughts  are  carried  back 
to  a  prior  period,  when  the  tree  grew  for  years  on  dry  land, 
enjoying  a  fit  soil  and  climate. 

It  has  been  already  remarked  that  there  are  rocks  in  the  inte- 
rior of  continents,  at  various  depths  in  the  earth,  and  at  great 
heights  above  the  sea,  almost  entirely  made  up  of  the  remains 
of  zoophytes  and  testacea.  Such  masses  may  be  compared  to 
modern  oyster-beds  and  coral  reefs ;  and,  like  them,  the  rate  of 
increase  must  have  been  extremely  gradual.  But  there  are  a 
variety  of  stony  deposits  in  the  earth's  crust  now  proved  to  have 
been  derived  from  plants  and  animals  of  which  the  organic  ori- 
gin was  not  suspected  until  of  late  years,  even  by  naturalists. 
Great  surprise  was  therefore  created  by  the  recent  discovery  of 
Professor  Ehrenberg  of  Berlin,  that  a  certain  kind  of  siliceous 
stone,  called  tripoli,  was  entirely  composed  of  millions  of  the 
skeletons  or  cases  of  microscopic  animalcules.  The  substance 
alluded  to  has  long  been  well  known  in  the  arts,  being  used  in 
the  form  of  powdef  for  polishing  stones  and  metals.  It  has  been 
procured,  among  other  places,  from  Bilin,  in  Bohemia,  where  a 
single  stratum,  extending  over  a  wide  area,  is  no  less  than  14 
feet  thick.  This  stone,  when  examined  with  a  powerful  micro- 
scope, is  found  to  consist  of  the  siliceous  cases  of  infusoria,  united 
together  without  any  visible  cement.  It  is  difficult  to  convey  an 

Fig.  14.  Fig.  15.  Fig.  16. 


These  figures  are  magnified  nearly  300  limes,  except  the  lower  figure  of  G.  ferru- 
ginea  (Fig.  16,  a,)  which^is  magnified  2000  times. 

idea  of  their  extreme  minuteness  ;  but  Ehrenberg  estimates  that 
in  the  Bilin  tripoli  there  are  41,000  millions  of  individuals  of  the 
Gaillonella  distans  (see  Fig.  15.)  in  every  cubic  inch,  which 
weighs  about  220  grains,  or  about  187  millions  in  a  single  grain. 
At  every  stroke,  therefore,  that  we  make  with  this  polishing 
powder,  several  millions,  perhaps  tens  of  millions,  of  perfect  fos- 
pils  are  crushed  to  atoms. 


40 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Infusoria  in  Tripoli Fossil  Infusoria. 


The  shells  or  shields  of  these  infusoria  are  of  pure  silex,  and 
their  forms  are  various,  but  very  marked  and  constant  in  partic- 
ular genera  and  species.  Thus,  in  the  family  Bacillaria,  (see 
Fig.  14.,)  the  fossil  species  preserved  in  tripoli  are  seen  to  exhi- 
bit the  same  divisions  and  transverse  lines  which  characterize  the 
living  shells  of  kindred  form.  With  these,  also,  the  siliceous 
spiculse  or  internal  supports  of  the  freshwater  sponge,  or  Span- 
gilla  of  Lamarck,  are  sometimes  intermingled  (see  the  needle- 
shaped  bodies  in  Fig.  18.)  These  flinty  cases  and  spiculae, 
although  hard,  are  very  fragile,  breaking  like  glass,  and  are 
therefore  admirably  adapted,  when  rubbed,  for  wearing  down 
into  a  fine  powder  fit  for  polishing  the  surface  of  metals. 


Fig.  18. 


Fig.  17. 


Besides  the  tripoli, 
which  is  formed  ex- 
clusively of  infusoria, 
there  occurs  in  the 
upper  part  of  the 
great  stratum  at  Bi- 
lin  another  heavier 
and  more  compact 
stone,  a  kind  of  se- 
mi-opal, in  which  in- 
numerable parts  of 
infusoria  and  spicu- 
lse of  the  Spongilla 
are  filled  with,  and 
cemented  together  by 
siliceous  matter.  It 
is  supposed  that  the 
shells  of  the  more 
delicate  animalcules 
have  been  dissolved 
by  water,  and  have 
thus  given  rise  to  this 

Fragment  of  semi-opal  from  the  great  bed  of  tripoli,  Bilin.   Opal,     in     which     the 

Fig.  17.  Natural  size.  more  durable  fossils 

Fig.  18.  The  same  magnified,  showing  circular  artic-are  preserved  like  in- 

ulations  of  a  species  of  Gaillonella,  and  spi-sects  in  amber.    This 

cu^  of  Spongilla.  opinion  is  confirmed 

by  the  fact  that  the  small  shells  decrease  in  number  and  shafp- 
ness  of  outline  in  proportion  as  the  opaline  cement  increases  in 
quantity. 

In  the  Bohemian  tripoli  above  described,  as  in  that  of  Planitz 


PART  I.     CHAPTER  III.  41 

Microscopic  Fossils Infusoria  in  Flints. 

in  Saxony,  the  species  of  infusoria  are  freshwater ;  but  in  other 
countries,  as  in  the  tripoli  of  the  Isle  of  France,  they  are  of  ma- 
rine species,  and  they  all  belong  to  formations  of  the  tertiary 
period,  which  will  be  spoken  of  hereafter.  (See  Part  II.) 

A  well-known  substance,  called  bog-iron  ore,  often  met  with 
in  peat  mosses,  has  also  been  shown  by  Ehrenberg  to  consist  of 
innumerable  articulated  threads,  of  a  yellow  ochre  colour,  com- 
posed partly  of  flint  and  partly  of  oxide  of  iron.  These  threads 
are  the  cases  of  a  minute  animalcule,  called  Gaillonella  fcrru- 
ginea  (Fig.  16.) 

It  is  clear  that  much  time  must  have  been  required  for  the 
accumulation  of  strata  to  which  countless  generations  of  infuso- 
ria have  contributed  their  shells ;  and  these  discoveries  lead  us 
naturally  to  suspect  that  other  deposits,  of  which  the  materials 
have  usually  been  supposed  to  be  inorganic,  may  in  reality  have 
been  derived  from  microscopic  organic  bodies.  That  this  is  the 
case  with  the  white  chalk,  has  often  been  imagined,  this  rock 
having  been  observed  to  abound  in  a  variety  of  marine  fossils, 
such  as  shells,  echini,  corals,  sponges,  Crustacea,  and  fishes. 
Mr.  Lonsdale,  on  examining  lately,  in  the  museum  of  the  Geo- 
logical Society  of  London,  portions  of  while  chalk  from  different 
parts  of  England,  found,  on  carefully  pulverizing  them  in  water, 
that  what  appear  to  the  eye  simply  as  white  grains  were,  in  fact, 
well-preserved  fossils.  He  obtained  about  a  thousand  of  these 
from  each  pound  weight  of  chalk,  some  being  fragments  of  mi- 
nute corallines,  others  entire  Foraminifera  and  Cytherinse.  The 
annexed  drawings  will  give  an  idea  of  the  beautiful  forms  of 

CylherincB  and  Foraminifera  from  the  chalk. 
Fig.  19.  Fig.  20.         Fig.  21.  Fig-.  22. 


Cytherina.  Portion  of  Lenticulina,  Lam.  Discorbis. 

jg§  Nodosaria.  (Operculina,  D'Orb.) 

many  of  these  bodies.  The  figures  a  a  represent  their  natural 
size,  but,  minute  as  they  seem,  the  smallest  of  them,  such  as  a, 
Fig.  22.,  are  gigantic  in  comparison  with  the  cases  of  infusoria 
before  mentioned.  There  is,  moreover,  good  reason  to  believe 
that  the  chambers  into  which  these  Foraminifera  are  divided  are 
actually  often  filled  with  hundreds  of  infusoria ;  for  many  of  the 
minute  grains  which  they  contain,  and  which  compose  the  envel- 
oping chalk,  have  been  observed,  under  a  powerful  microscope, 
to  consist  of  circular  discs,  like  the  articulations  of  Gaillonella, 
before  represented  in  Fig,  18.  The  bodies  alluded  to  were  cal- 

D  * 


42       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Slow  Deposition  of  Strata. 


careous ;  but  Ehrenberg  has  discovered  others  in  the  flints  of  the 
chalk,  which,  like  the  infusoria  in  tripoli,  are  siliceous.  These 
forms  are  especially  apparent  in  the  white  coating  of  flints,  often 
accompanied  by  innumerable  needle-shaped  spicula?  of  sponges : 
and  the  same  are  occasionally  visible  in  the  central  parts  of 
chalk  flints  where  they  are  of  a  lighter  colour.  After  reflecting 
on  these  discoveries,  we  are  naturally  led  on  to  conjecture  that, 
as  the  formless  cement  in  the  semi-opal  of  Bilin  has  been  de- 
rived from  the  decomposition  of  animal  remains,  so  also  even 
those  parts  of  chalk  flints  in  which  no  organic  structure  can  be 
recognized  may  nevertheless  have  constituted  a  part  of  micro- 
scopic animalcules. 

"  The  dust  we  tread  upon  was  once  alive  !" — Byron. 

How  faint  an  idea  does  this  exclamation  of  the  poet  convey  of 
the  real  wonders  of  nature !  for  here  we  discover  proofs  that  the 
calcareous  and  siliceous  dust  of  which  hills  are  composed  has 
not  only  been  once  alive,  but  almost  every  particle,  albeit  invisi- 
ble to  the  naked  eye,  still  retains  the  organic  structure  which,  at 
periods  of  time  incalculably  remote,  was  impressed  upon  it  by 
the  powers  of  life. 

As  1  have  dwelt  upon  the  proofs  of  the  slowness  with  which 
fossiliferous  strata  in  general  have  been  produced,  I  may  remark 
that  some  writers  have  argued,  from  the  appearances  of  certain 
deposits  containing  coal,  that  sedimentary  rocks  of  great  thick- 
ness have  been  accumulated  with  rapidity.  This  conclusion  has 
been  drawn  chiefly  from  a  remarkable  phenomenon, — the  posi- 
tion of  the  trunks  of  fossil  trees  intersecting  obliquely,  and  often 
at  right  angles,  the  planes  of  many  strata.  For  a  full  examina- 
tion of  this  question,  the  reader  is  referred  to  the  chapter  on  the 
carboniferous  formations,  in  the  sequel ;  and  I  shall  merely  say 
here,  that,  although  partial  deposits  have  been  thrown  down  in 
the  spots  where  these  fossil  trees  occur  in  a  comparatively  short 
lapse  of  time,  yet  we  can  by  no  means  infer  that  a  similar  rate 
of  increase  of  carboniferous  rocks  prevailed  simultaneously  over 
a  wide  area.  On  the  other  hand,  the  vegetable  origin  of  coal  is 
now  universally  admitted  by  geologists ;  and,  when  we  discuss 
the  probable  manner  in  which  the  terrestrial  plants  from  which 
it  was  derived  were  imbedded  in  marine  shale  and  sandstone,  we 
shall  find  it  necessary  to  suppose  a  long  succession  of  operations. 

Freshwater  and  marine  fossils. — Strata,  whether  deposited 
in  salt  or  fresh  ivater,  have  the  same  forms ;  but  the  fossils  are 
very  different  in  the  two  cases,  for  the  same  reason  that  aquatic 
animals  which  frequent  lakes  and  rivers  are  distinct  from  those 
inhabiting  the  sea.  As  an  example  of  English  strata  character- 
ized by  freshwater  fossils,  I  may  point  out  a  formation  which 


PART  I.     CHAPTER  III.  43 


Distinction  of  Freshwater  from  Marine  Formations. 

extends  over  the  northern  part  of  the  Isle  of  Wight,  composed 
of  marl  and  limestone  more  than  fifty  feet  thick.  The  shells  are 
principally,  if  not  all,  of  extinct  species ;  but  they  are  of  the 
same  genera  as  those  now  abounding  in  ponds  and  lakes,  either 
in  our  own  country  or  warmer  latitudes. 

In  many  parts  of  France,  as  in  Auvergne,  for  example,  strata 
of  limestone,  marl,  and  sandstone  occur,  hundreds  of  feet  thick, 
which  contain  exclusively  freshwater  and  land  shells,  together 
with  the  remains  of  terrestrial  quadrupeds.  The  number  of  land 
shells  scattered  through  some  of  these  freshwater  deposits  is 
exceedingly  great ;  and  there  are  even  districts  where  the  rocks 
scarcely  contain  any  other  fossils  except  snail-shells  (helices  ;) 
as,  for  instance,  the  limestone  on  the  left  bank  of  the  Rhine, 
between  Mayence  and  Worms,  at  Oppenheim,  Findheim,  Buden- 
heim,  and  other  places.  In  order  to  account  for  this  phenome- 
non, the  geologist  has  only  to  examine  the  small  deltas  of  tor- 
rents which  enter  the  Swiss  lakes  when  the  waters  are  low,,  such 
as  the  newly-formed  plain  where  the  Kander  enters  the  Lake  of 
Thun.  He  there  sees  sand  and  mud  strewed  over  with  innume- 
rable dead  land  shells,  which  have  been  brought  down  from  val- 
leys in  the  Alps  in  the  preceding  spring,  during  the  melting  of 
the  snows.  Again,  if  we  search  the  sands  on  the  borders  of  the 
Rhine,  in  the  lower  part  of  its  course,  we  find  countless  land 
shells  mixed  with  others  of  species  belonging  to  lakes,  stagnant 
pools,  and  marshes.  These  individuals  have  been  washed  away 
from  the  alluvial  plains  of  the  great  river  and  its  tributaries, 
some  from  mountainous  regions,  others  from  the  low  country. 

Although  freshwater  formations  are  often  of  great  thickness, 
yet  they  are  usually  very  limited  in  area  when  compared  to 
marine  deposits,  just  as  lakes  and  estuaries  are  of  small  dimen- 
sions in  comparision  with  seas. 

We  may  distinguish  a  freshwater  formation,  first,  by  the 
absence  of  many  fossils  almost  invariably  met  with  in  marine 
strata.  For  example,  there  are  no  corals,  no  sea-urchins,  and 
scarcely  any  other  zoophytes ;  no  chambered  shells,  such  as  the 
nautilus,  nor  microscropic  Foraminifera.  But  it  is  chiefly  by 
attending  to  the  forms  of  the  mollusca  that  we  are  guided  in 
determining  the  point  in  question.  In  a  freshwater  deposit,  the 
number  of  individual  shells  is  often  as  great,  if  not  greater,  than 
in  a  marine  stratum ;  but  there  are  fewer  species  and  genera. 
This  might  be  anticipated  from  the  fact  that  the  genera  and  spe- 
cies of  recent  freshwater  and  land  shells  are  few  when  contrasted 
with  the  marine.  Thus,  the  genera  of  true  mollusca  according 
to  Blainville's  system,  excluding  those  of  extinct  species  and 
those  without  shells,  amount  to  about  200  in  number,  of  which 


44 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Distinction  of  Freshwater  from  Marine  Formations. 


the  terrestrial  and  freshwater  genera  scarcely  form  more  than  a 
sixth. 

Almost  all  bivalve  shells,  or  those  of  acephalous  mollusca,  are 
marine,  about  ten  only  out  of  ninety  genera  being  freshwater. 
Among  these  last,  the  four  most  common  forms,  both  recent  and 
fossil,  are  Cyclas,  Cyrena,  Unio,  and  Anodonta  (see  figures ;) 
the  two  first  and  two  last  of  which  are  so  nearly  allied  as  to  pass 
into  each  other. 


Fig.  23. 


Fig.  24. 


Cyclas  obanoata ;  fossil.    Hants. 

Fig.  25. 


Cyrena  trigonula ;  fossil.    Grays,  Essex. 

Fig.  26.  Fig.  27. 


Anodonta  Cordierii; 
fossil.    Paris. 


dnodonta  latimargi- 
natus;  recent.    Bahia. 


Unio  littoralis  ; 
recent.    Auvergne. 


Lamarck  divided  the  bivalve  mollusca  into  the  Dimyary,  or 
those  having  two  large  muscular  impressions  in  each  valve,  as 
ab  in  the  Cyclas,  Fig.  23.,  and  the  Monomyary,  such  as  the  oys- 


Fig.  28. 


Ostrea  vesicularis  (Orypht 
globosa,  Sow.) ;  chalk. 

nia,  and  Neritina. 


ter  and  scallop,  in  which  there  is  only  one 
of  these  impressions,  as  is  seen  in  Fig.  28. 
Now,  as  none  of  these  last,  or  the  ummus- 
cular  bivalves,  are  freshwater,  we  may  at 
once  presume  a  deposit  in  which  we  find 
any  of  them  to  be  marine. 

The  univalve  shells  most  characteristic 
of  freshwater  deposits  are,  Planorbis,  Lim- 
nea,  and  Paludina.  (See  figures.)  But  to 

« these  are  occasionally  added  Physa,  Succi- 
nea,  Ancylus,  Valvata,  Melanopsis,  Mela- 

(See  figures.) 


PART  I.     CHAPTER  III. 


45 


Distinction  of  Freshwater  from  Marine  Formations. 


Fig.  29. 


Fig.  30. 


Fig.  31. 


Planorbis  euomphalus ; 
fossil.    Isle  of  Wight. 

Fig.  32. 


Succinea  elovgnta  ; 
fossil.    Loess,  Rhine. 


Fig.  36. 


Limnea  longiscata  ; 
fossil.    Hants. 


Paludina  lento, ; 
fossil.    Hants 


Fig.  33. 


Fig.  34.      Fig.  a5. 


Ancylus  elegans ; 
fossil.    Hants. 


Valvata ;  Physa 

fossil.  hypnorum, 

Grays,  Essex.       recent. 


Fig.  37. 


Fig.  38. 


Fig.  39. 


Auricula  ; 
recent.    Ava. 


Melanopsis  buc- 

cinoidea;  recent. 

Asia. 


In  regard  to  one  of  these,  the  Ancylus  (Fig.  33.,)  Mr.  Gray 
observes  that  it  sometimes  differs  in  no  respect  from  the  marine 
Siphonaria,  except  in  the  animal.  The  shell,  however,  of  the 
Ancylus  is  usually  thinner.* 

Some 'naturalists  include  Neritina  (Fig.  40.)  and  the  marine 
Nerita  (Fig.  41.)  in  the  same  genus,  it  being  scarcely  possible 
Fig.  40.  Fig.  41. 


Neritina  globulus.    Paris  basin.          Jftrita  granulosa.      Paris  basin. 


*  Gray,  Phil.  Trans.  1835,  p.  302. 


46 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Distinction  of  Freshwater  from  Marine  Formations. 


Fi  42  to  distinguish  tne  two  °y  S°°d  generic  characters. 
But,  as  a  general  rule,  the  fluviatile  species  are  small- 
er, smoother,  and  more  globular  than  the  marine; 
and  they  have  never,  like  the  Neritse,  the  inner  mar- 

fin   of  the   outer  lip   toothed  or   crenulated.     (See 
ig.  41.) 

A  few  genera,  among  which  Cerithium  (Fig.  42.) 
is  the  most  abundant,  are  common  both  to  rivers  and 
the  sea,  having  species  peculiar  to  each.  Other 
genera,  like  Auricula  (Fig.  36.),  are  amphibious,  liv- 
ing both  in  freshwater  and  on  land. 

The  terrestrial  shells  are  all  univalves.  The  most 
abundant  genera  among  these,  both  in  a  recent  and 
fossil  state,  are  Helix  (Fig.  45.),  Cyclostoma,  Pupa, 
(Fig.  44.),  Clausilia,  Bulimias  (Fig.  43.),  and  Acha- 
^na  »  which  two  last  are  nearly  allied  and  pass  into 
each  other.  The  same  may  be  said  with  almost 
equal  truth  of  Pupa  and  Clausilia. 
Fig.  43.  Fig.  44.  Fig.  45. 


Paris  basin. 


Pupa  muscorum. 


Helix  plebcium. 


Fig.  46. 


Bulimus 
lubricus. 

Att  recent  ;  and  also  fossil  from  Loess  of  Rhine. 

The  Ampullaria  (Fig.  46.)  is  another  genus  of  shells,  inhabit- 
ing rivers  and  ponds  in  hot  countries.  Many 
fossil  species  have  been  referred  to  this  genus, 
but  they  have  been  found  chiefly  in  marine  for- 
mations, and  are  suspected  by  some  concho- 
logists  to  belong  to  Natica  and  other  marine 
genera. 

All  univalve  shells  of  land  and  freshwater 
species  have  entire  mouths  ;  .and  this  circum- 
stance  may  often  serve  as  a  convenient  rule  for 
distinguishing  freshwater  from  marine  strata; 
since,  if  any  univalves  occur  of  which  the  mouths  are  not  entire, 
we  may  conclude  that  the  formation  is  marine.  The  aperture 
is  said  to  be  entire  in  such  shells  as  the  Ampullaria  and  the  land 
shells  figured  in  this  page,  when  its  outline  is  not  interrupted  by 
an  indentation  or  notch'such  as  that  in  Ancillaria  (Fig.  48.)  ;  or 
is  not  prolonged  into  a  canal,  as  that  seen  at  a  in  Pleuromota 
(Fig.  47.). 

The  mouths  of  a  large  proportion  of  the  marine  univalves 


from  the  Jumna, 


PART  I.     CHAPTER  III. 


47 


Distinction  of  Freshwater  from  Marine  Formations. 


Fig.  47. 


Fig.  48. 


Pleurotomata 

rotata. 

Subap.  hills, 

Italy. 


Jlncillaria  sululata.    London  clay. 


have  either  these  notches  or  canals,  and  all  these  species  are, 
without  exception,  carnivorous  ;  whereas  nearly  all  testacea  hav- 
ing entire  mouths,  are  plant-eaters,  whether  the  species  be 
marine,  freshwater,  or  terrestrial. 

There  is,  however,  one  genus  which  affords  an  occasional 
exception  to  one  of  the  above  rules.  The  Cerithium  (Fig.  42.,) 
although  provided  with  a  short  canal,  comprises  some  species 
which  inhabit  salt,  others  brackish,  and  others  fresh  water. 

Among  the  fossils  very  common  in  freshwater  deposits,  are 
the  shells  of  Cypris,  a  minute  crustaceous  animal,  having  a  shell 
much  resembling  that  of  the  bivalve  mollusca.*  Many  minute 
living  species  of  this  genus  swarm  in  lakes  and  stagnant  pools  in 
Great  Britain ;  but  their  shells  are  not,  if  considered  separately, 
conclusive  as  to  the  freshwater  origin  of  a  deposit,  because 
another  kindred  genus  of  the  same  order,  the  Cytherina  of 
Fig.  49.  Fig.  50. 


Chara  medicaginula  ;  Chara  elastica ;  recent.     Italy. 

fossil.  Isle  of  Wight,  a,  Sessile  seed-vessel  between  the  division  of   the 

a,  Seed-vesels,  leaves  of  the  female  plant. 

magnified  20  b,  Transverse  section  of  a  branch,  with  five  seed- 
diameters,  vessels  magnified,  seen  from  below  upwards. 

6,  Stem  magnified. 


*  See  figures  in  chap,  on  Wealden,  Port  II. 


48      LYELL'S  ELEMENTS  OF  GEOLOGY. 


Distinction  of  Freshwater  from  Marine  Formations. 


Lamarck  (see  Fig.  19.  p.  41.,)  inhabits  salt  water;  and,  although 
the  animal  differs  slightly,  the  shell  is  undistinguishable  from  that 
of  the  Cypris. 

The  seed-vessels  of  Chara,  a  genus  of  aquatic  plants,  are 
very  frequent  in  freshwater  strata.  These  seed-vessels  were 
called,  before  their  true  nature  was  known,  gyrogonites,  and 
were  supposed  to  be  shells.  (See  Fig.  49.  a.) 

The  Charse  inhabit  the  bottom  of  lakes  and  ponds,  and 
flourish  mostly  where  the  water  is  charged  with  carbonate  of 
lime.  Their  seed-vessels  are  covered  with  a  very  tough  integu- 
ment, capable  of  resisting  decomposition;  to  which  circumstance 
we  may  attribute  their  abundance  in  a  fossil  state.  The  annexed 
figure  (Fig.  50.)  represents  a  branch  of  one  of  many  new  spe- 
cies found  by  Professor  Amici  in  the  lakes  of  northern  Italy. 
The  seed-vessel  in  this  plant  is  more  globular  than  in  the  British 
Charse,  and  therefore  more  nearly  resembles  in  form  the  extinct 
fossil  species  found  in  England,  France,  and  other  countries. 
The  stems,  as  well  as  the  seed-vessels,  of  these  plants  are  found 
both  in  modern  shell  marl  and  in  ancient  freshwater  formations. 
They  are  generally  composed  of  a  large  tube  surrounded  by 
smaller  tubes ;  the  whole  stem  being  divided  at  certain  intervals 
by  transverse  partitions  or  joints.  (See  6,  Fig.  49.) 

It  is  not  uncommon  to  meet  with  layers  of  vegetable  matter, 
impressions  of  leaves,  and  branches  of  trees,  in  strata  containing 
freshwater  shells ;  and  we  also  find  occasionally  the  teeth  and 
bones  of  land  quadrupeds,  of  species  now  unknown.  The  man- 
ner by  which  such  remains  are  occasionally  carried  by  rivers 
into  lakes,  especially  during  floods,  has  been  fully  treated  of  in 
the  "  Principles  of  Geology."* 

The  remains  of  the  fish  are  occasionally  useful  in  determining 
the  freshwater  origin  of  strata.  Certain  genera,  such  as  carp, 
perch,  pike,  and  loach,  (Cyprinus,  Perm,  Esox,  and  Cobitis,) 
as  also  Lebias,  being  peculiar  to  freshwater.  Other  genera  con- 
tain some  freshwater  and  some  marine  species,  as  Coitus,  Mugil, 
and  Anguilla,  or  eel.  The  rest  are  either  common  to  rivers 
and  the  sea,  as  the  salmon  ;  or  are  exclusively  characteristic  of 
salt  water.  The  above  observations  respecting  fossil  fishes  are 
applicable  only  to  the  more  modern  or  tertiary  deposits ;  for  in 
the  more,  ancient  rocks  the  forms  depart  so  widely  from  those  of 
existing  fishes,  that  it  is  very  difficult,  at  least  in  the  present  state 
of  science,  to  derive  any  information  from  icthyolites,  respecting 
the  element  in  which  strata  were  deposited. 

j£ ( 

*See  Index,  "  Fossil ization." 


PART  I.     CHAPTER  I 


Alternations  of  Marine  and  Freshwater 


The  alternation  of  marine  and  freshwater 
a  small  and  large  scale,  are  facts  well  ascertained 
When  it  occurs  on  a  small  scale,  it  may  have  arisen  from  the 
alternate  occupation  of  certain  spaces  by  river-water  and  the 
sea ;  for  in  the  flood  season  the  river  forces  back  the  ocean  and 
freshens  it  over  a  large  area,  depositing  at  the  same  time  its  sedi- 
ment ;  after  which  the  salt  water  again  returns,  and,  on  resum- 
ing its  former  place,  brings  with  it  sand,  mud,  and  marine  shells. 

There  are  also  lagoons  at  the  mouths  of  many  rivers,  as  the 
Nile  and  Mississippi,  which  are  divided  off  by  bars  of  sand  from 
the  sea,  and  which  are  filled  with  salt  and  fresh  water  by  turns. 
They  often  communicate  exclusively  with  the  river  for  months, 
years,  or  even  centuries ;  and  then  a  breach  being  made  in  the 
bar  of  sand,  they  are  for  long  periods  filled  with  salt  water. 

The  Lym-Fiord  in  Jutland  offers  an  excellent  illustration  of 
analogous  changes ;  for,  in  the  course  of  the  last  thousand 
years,  the  western  extremity  of  this  long  frith,  which  is  120 
miles  in  length,  including  its  windings,  has  been  four  times  fresh 
and  four  times  salt,  a  bar  of  sand  between  it  and  the  ocean  hav- 
ing been  as  often  formed  and  removed.  The  last  irruption  of 
salt  water  happened  in  1824,  when  the  North  Sea  entered,  kill- 
ing all  the  freshwater  shells,  fish,  and  plants ;  and  from  that 
time  to  the  present,  the  sea-weed  Fucus  vesiculosus,  together 
with  oysters  and  other  marine  mollusca,  have  succeeded  the 
Cyclas,  Limnea,  Paludina,  and  Charre.* 

But  changes  like  these  in  the  Lym-Fiord,  and  those  before- 
mentioned  as  occurring  at  the  mouths  of  great  rivers,  will  only 
account  for  some  cases  of  marine  deposits  resting  on  freshwater 
strata.  When  we  find,  as  in  the  south-east  of  England,  a  great 
series  of  freshwater  beds,  resting  upon  one  marine  formation  of 
great  thickness,  and  again  covered  by  another  more  than  1000 
feet  thick,  we  shall  find  it  necessary  to  seek  for  a  different  expla- 
nation of  the  phenomena.f 


*  See  Principles  of  Geology,  Index,  "  Lym-Fiord." 
t  See  account  of  Wealden,  Part  II. 


50       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Consolidation  of  Strata. 


CHAPTER  IV. 


CONSOLIDATION   OF   STRATA  AND   PETRIFACTION   OF   FOSSILS. 

Chemical  and  mechanical  deposits — Cementing  together  of  particles — Hard- 
ening by  exposure  to  air — Concretionary  nodules — Consolidating  effects  of  pres- 
sure—Mineralisation  of  organic  remains — Impressions  and  casts  how  formed — 
Fossil  wood — Goppert's  experiments — Precipitation  of  stony  matter  most  rapid 
where  putrefaction  is  going  on — Source  of  lime  in  solution — Silex  derived  from 
decomposition  of  felspar — Proofs  of  the  lapidification  of  some  fossils  soon  after 
burial,  of  others  when  much  decayed. 

HAVING  spoken  in  the  preceding  chapters  of  the  forms  of 
stratification,  both  as  dependent  on  the  deposition  of  inorganic 
matter  and  the  distribution  of  fossils,  I  may  next  treat  of  the 
consolidation  of  stratified  rocks,  and  the  petrifaction  of  imbedded 
organic  remains. 

Chemical  and  mechanical  deposits. — A  distinction  has  been 
made  by  geologists  between  deposits  of  a  chemical,  and  those 
of  a  mechanical,  origin.  By  the  latter  name  are  designated  beds 
of  mud,  sand,  or  pebbles  produced  by  the  action  of  running 
water,  also  accumulations  of  stones  and  scoriae  thrown  out  by  a 
'volcano,  which  have  fallen  into  their  present  place  by  the  force 
of  gravitation.  But  the  matter  which  forms  a  chemical  deposit 
has  not  been  mechanically  suspended  in  water,  but  in  a  state  of 
solution  until  separated  by  chemical  action. ;  In  this  manner  car- 
bonate of  lime  is  often  thrown  to  the  bottom  of  lakes  and  seas 
in  a  solid  form,  as  may  be  well  seen  in  many  parts  of  Italy, 
where  mineral  springs  abound,  and  where  the  calcareous  stone, 
called  travertin,  is  deposited.  In  these  springs  the  lime  is  usually 
held  in  solution  by  an  excess  of  carbonic  acid,  or  by  heat  if  it 
be  a  hot  spring,  until  the  water,  on  issuing  from  the  earth,  cools 
or  loses  part  of  its  acid.  The  calcareous  matter  then  falls  down 
in  a  solid  state,  encrusting  shells,  fragments  of  wood  and  leaves, 
and  binding  them  together.* 

In  coral  reefs,  large  masses  of  limestone  are  formed  by  the 
stony  skeletons  of  zoophytes ;  and  these;  together  with  shells, 
become  cemented  together  by  carbonate  of  lime,  part  of  which 
is  probably  furnished  to  the  sea-water  by  the  decomposition  of 
dead  corals.  Even  shells  of  which  the  animals  are  still  living, 

*  See  Principles  of  Geology,  Index,  "  Calcareous  Springs,"  &c 


PART  I.     CHAPTER  IV.  51 


Consolidation  of  Strata. 


on  these  reefs,  are  very  commonly  found  to  be  encrusted  over 
with  a  hard  coating  of  limestone.* 

If  sand  and  pebbles  are  carried  by  a  river  into  the  sea,  and 
these  are  bound  together  immediately  by  carbonate  of  lime,  the 
deposit  may  be  described  as  of  a  mixed  origin,  partly  chemical, 
and  partly  mechanical. 

Now,  the  remarks  already  made  in  Chapter  II.  on  the  original 
horizontally  of  stratd  are  strictly  applicable  to  mechanical  depo- 
sits, and  only  partially  to  those  of  a  mixed  nature.  Such  as  are 
purely  chemical  may  be  formed  on  a  very  steep  slope,  or  may 
even  encrust  the  vertical  walls  of  a  fissure,  and  be  of  equal 
thickness  throughout. ;  but  such  deposits  are  of  small  extent,  and 
for  the  most  part  confined  to  vein-stones. 

Cementing  of  particles. — It  is  chiefly  in  the  case  of  calca- 
reous rocks  that  solidification  takes  place  at  the  time  of  deposi- 
tion. But  there  are  many  deposits  in  which  a  cementing  process 
comes  into  operation  long  subsequently.  We  may  sometimes 
observe,  where  the  water  of  ferruginous  or  calcareous  springs 
has  flowed  through  a  bed  of  sand  or  gravel,  that  iron  or  carbo- 
nate of  lime  has  been  deposited  in  the  interstices  between  the 
grains  or  pebbles,  so  that  in  certain  places  the  whole  has  been 
bound  together  into  a  stone,  the  same  set  of  strata  remaining  in 
other  parts  loose  and  incoherent. 

Proofs  of  a  similar  cementing  action  are  seen  in  a  rock  at 
Kelloway  in  Wiltshire.  A  peculiar  band  of  sandy  strata,  belong- 
ing to  the  group  called  Oolite  by  geologists,  may  be  traced 
through  several  counties,  the  sand  being  for  the  most  part  loose 
and  unconsolidated,  but  becoming  stony  near  Kelloway.  In  this 
district  there  are  numerous  fossil  shells  which  have  decomposed, 
having  for  the  most  part  left  only  their  casts.  The  calcareous 
matter  hence  derived  has  evidently  served,  at  some  former  period, 
as  a  cement  to  the  siliceous  grains  of  sand,  and  thus  a  solid 
sandstone  has  been  produced.  If  we  take  fragments  of  many 
other  argillaceous  grits,  retaining  the  casts  of  shells,  and  plunge 
them  into  dilute  muriatic  or  other  acid,  we  see  them  immediately 
changed  into  common  sand  and  mud;  the  cement  of  lime, 
derived  from  the  shells,  having  been  dissolved  by  the  acid. 

Traces  of  impressions  and  casts  are  often  extremely  faint.  In 
some  loose  sands  of  recent  date  we  meet  with  shells  in  so 
advanced  a  stage  of  decomposition  as  to  crumble  into  powder 
when  touched.  It  is  clear  that  water  percolating  such  strata  may 
soon  remove  the  calcareous  matter  of  the  shell ;  and,  unless 
circumstances  cause  the  carbonate  of  lime  to  be  again  deposited, 

*  See  Principles  of  Geology,  Index,  "Travertin,"  "Coral  reefs,"  &c. 


52       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Consolidation  of  Strata. 


the  grains  of  sand  will  not  be  cemented  together ;  in  which  case 
no  memorial  of  the  fossil  will  remain.  The  absence  of  organic 
remains  from  many  aqueous  rocks  may  be  thus  explained. 

In  some  conglomerates,  like  the  puddingstone  of  Hertfordshire, 
flinty  pebbles  and  sand  are  united  by  a  siliceous  cement  so  firmly, 
that  if  a  block  be  fractured  the  rent  passes  as  readily  through 
the  pebbles  as  through  the  cement. 

It  is  probable  that  many  strata  became  solid  at  the  time  when 
they  emerged  from  the  waters  in  which  they  were  deposited,  and 
when  they  first  formed  a  part  of  the  dry  land.  A  well-known 
fact  seems  to  confirm  this  idea ;  by  far  the  greater  number  of  the 
stones  used  for  building  and  road-making  are  much  softer  when 
first  taken  from  the  quarry  than  after  they  have  been  long 
exposed  to  the  air.  Hence  it  is  found  desirable  to  shape  the 
stones  which  are  to  be  used  in  architecture  while  they  are  yet 
soft  and  wet,  and  whil§  they  contain  their  "  quarry-water,"  as  it 
is  called ;  also  to  break  up  stone  intended  for  roads  when  soft, 
and  then  leave  it  to  dry  in  the  air  for  months  that  it  may  harden. 
Such  induration  may  perhaps  be  accounted  for  by  supposing  the 
water,  which  penetrates  the  minutest  pores  of  rocks,  to  deposit 
on  evaporation  carbonate  of  lime,  iron,  silex,  and  other  minerals 
previously  held  in  solution.  These  particles,  on  crystallizing, 
would  not  only  be  deprived  themselves  of  freedom  of  motion, 
but  would  also  bind  together  other  portions  of  the  rock  which 
before  were  loosely  aggregated.  On  the  same  principle  wet  sand 
and  mud  become  as  hard  as  stone  when  frozen ;  because  one 
ingredient  of  the  mass,  namely,  the  water,  has  crystallized,  so  as 
to  hold  firmly  together  all  the  separate  particles  of  which  the 
loose  mud  and  sand  were  composed. 

Dr.  MacCulloch  mentions  a  sandstone  in  Sky,  which  may  be 
moulded  like  dough  when  first  found  ;  and  another  from  China, 
which  is  compressible  by  the  hand  when  immersed  in  water.  But 
it  is  not  merely  these  compounds  which  readily  admit  water  to 
penetrate  into  them  ;  some  simple  minerals,  says  the  same  wri- 
ter, which  are  rigid  and  as  hard  as  glass  in  our  cabinets,  are 
often  flexible  and  soft  in  their  native  beds ;  this  is  the  case  with 
asbestos,  sahlite,  tremolite,  and  calcedony,  and  it  is  reported  also 
to  happen  in  the  case  of  the  beryl.* 

The  marl  recently  deposited  at  the  bottom  of  Lake  Superior, 
in  North  America,  is  soft,  and  often  filled  with  fresh- water  shells ; 
but  if  a  piece  be  taken  up  and  dried,  it  becomes  so  hard  that  it 
can  only  be  broken  by  a  smart  blow  of  the  hammer.  If  the  lake 
therefore  was  drained,  such  a  deposit  would  be  found  to  consist 

*  Dr.  MacCulloch,  Syst.  of  Geol.  vol.  i.  p.  123. 


PART  1.    CHAPTER  IV.  53 

Concretionary  Structure  in  Stratified  Rocks. 

of  strata  of  marlstone,  like  that  observed  in  many  ancient  Euro- 
pean formations,  and  like  them  containing  freshwater  shells.* 

It  is  probable  that  some  of  the  heterogeneous  materials  which 
rivers  transport  to  the  sea  may  at  once  set  under  water,  like  the 
artificial  mixture  called  pozzolana,  which  consists  of  fine  volca- 
nic sand  charged  with  about  20  per  cent,  of  iron,  and  the  addi- 
tion of  a  small  quantity  of  lime.  This  substance  hardens  and 
becomes  a  solid  stone  in  water,  and  was  used  by  the  Romans  in 
constructing  the  foundations  of  buildings  in  the  sea. 

Consolidation  in  these  cases  is  brought  about  by  the  action  of 
chemical  affinity  on  finely  comminuted  matter  previously  sus- 
pended in  water.  After  deposition  similar  particles  seem  to  exert 
a  mutual  attraction  on  each  other,  and  congregate  together  in 
particular  spots,  forming  lumps,  nodules,  and  concretions.  Thus 
in  many  argillaceous  deposits  there  are  calcareous  balls,  or  sphe- 
rical concretions,  ranged  in  layers  parajlel  to  the  general  strati- 
fication ;  an  arrangement  which  took  place  after  the  shale  or 
marl  had  been  thrown  down  in  successive  laminse ;  for  these 
p.  -,  laminse  are  often  traced  in  the 

°'      __^  concretions,  remaining  parallel  to 

those  of  the  surrounding  uncon- 
solidated  rock.  (See  Fig.  51.) 
Such  nodules  of  limestone  have 
often  a  shell  or  other  foreign 
Calcareous  nodules  in  Lias.  body  in  the  centre.f 

Among  the  most  remarkable  examples  of  concretionary 
structure  are  those  described  by  Professor  Sedgwick  as  abound- 
ing in  the  magnesian  limestone  of  the  north  of  England.  The 
spherical  balls  are  of  various  sizes,  from  that  of  a  pea  to  a 
diameter  of  several  feet,  and  they  have  both  a  concentric  and 
radiated  structure,  while  at  the  same  time  the  laminse  of  original 
deposition  pass  uninterruptedly  through  them.  In  some  cliffs 
this  limestone  resembles  a  great  irregular  pile  of  cannon-balls. 
Some  of  the  globular  masses  have  their  centre  in  one  stratum, 
while  a  portion  of  their  exterior  passes  through  to  the  stratum 
above  or  below.  Thus  the  larger  spheroid  in  the  annexed  sec- 
Fig.  52.  tion  (Fig.  52.)  passes  from  the  stra- 
tum b  upwards  into  a.  In  this 
instance  we  must  suppose  the  depo- 
sition of  a  series  of  minor  layers, 
first  forming  the  stratum  &,  and 
afterwards  the  incumbent  stratum  a; 

Spheroidal  concKtwns^inmagncsian     ^^    &    movement    of    the    particles 

*  Principles  of  Geology,  Index,  "  Superior,  Lake." 
t  See  De  la  Beche's  Geological  Researches,  p.  95. 


54       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Consolidation  of  Strata  by  Pressure  and  Heat. 


took  place,  and  the  carbonates  of  lime  and  magnesia  separated 
from  the  more  impure  and  mixed  matter  forming  the  still  unconso- 
lidated  parts  of  the  stratum.  Crystallization,  beginning  at  the 
centre,  must  have  gone  on  forming  concentric  coats  around  the 
original  nucleus,  without  interfering  with  the  laminated  structure 
of  The  rock.  As  to  the  radiations  from  a  centre,  it  is  a  phenome- 
non which,  however  singular,  is  common  in  spherical  concretions 
of  various  mineral  ingredients. 

When  the  particles  of  rocks  have  been  thus  re-arranged  by 
chemical  forces,  it  is  sometimes  difficult  or  impossible  to  ascer- 
tain whether  certain  lines  of  division  are  due  to  original  deposi- 
tion or  to  the  subsequent  aggregation  of  similar  particles. 

p.     _„  Thus  suppose  three  strata  of  grit, 

A,  B,  C,  are  charged  unequally  with 
f  calcareous  matter,  and  that  B  is  the 


III 


Bi- 


most  calcareous.  If  consolidation 
takes  place  in  B,  the  concretionary 
action  may  spread  upwards  into  a 
part  of  A,  where  the  carbonate  of  lime  is  more  abundant  than  in 
the  rest ;  so  that  a  mass,  d,  e,  jf,  forming  a  portion  of  the  supe- 
rior stratum,  becomes  united  with  B  into  one  solid  mass  of  stone. 
The  original  line  of  division  df,  e,  being  thus  effaced,  the  line, 
d,f,  would  generally  be  considered  as  the  surface  of  the  bed  B, 
though  not  strictly  a  true  plane  of  stratification. 

Pressure  and  heat. — When  sand  and  mud  sink  to  the  bottom 
of  a  deep  sea,  the  particles  are  not  pressed  down  by  the  enormous 
weight  of  the  incumbent  ocean ;  for  the  water,  which  becomes 
mingled  with  the  sand  and  mud,  resists  pressure  with  a  force 
equal  to  that  of.  the  column  of  fluid  above.  The  same  happens 
in  regard  to  organic  remains  which  are  filled  with  water  under 
great  pressure  as  they  sink,  otherwise  they  would  be  immediately 
crushed  to  pieces  and  flattened.  Nevertheless,  if  the  materials 
of  a  stratum  remain  in  a  yielding  state,  and  do  not  set  or  solidify, 
they  will  be  gradually  squeezed  down  by  the  weight  of  other 
materials  successively  heaped  upon  them,  just  as  soft  clay  or 
loose  sand  on  which  a  house  is  built  may  give  way.  By  such 
downward  pressure  particles  of  clay,  sand,  and  marl  may  become 
packed  into  a  smaller  space,  and  be  made  to  cohere  together 
permanently. 

Analogous  effects  of  condensation  may  arise  when  the  solid 
parts  of  the  earth's  crust  are  forced  in  various  directions  by  those 
mechanical  movements  afterwards  to  be  described,  by  which 
strata  have  been  bent,  broken,  and  raised  above  the  level  of  the 
sea.  Rocks  of  more  yielding ,  materials  must  often  have  been 


PART  I.     CHAPTER  IV. 


55 


Mineralization  of  Organic  Remains. 


forced  against  others  previously  consolidated,  and,  thus  com- 
pressed, may  have  acquired  a  new  structure. 

But  the  action  of  heat  at  various  depths  in  the  earth  is  proba- 
bly the  most  powerful  of  all  causes  in  hardening  sedimentary 
strata.  To  this  subject  I  shall  refer  again  when  treating  of  the 
metamorphic  rocks,  and  of  the  slaty  and  jointed  structure. 

Mineralization  of  organic  remains. — The  changes  which 
fossil  organic  bodies  have  undergone  since  they  were  first  imbed- 
ded in  rocks,  throw  much  light  on  the  consolidation  of  strata. 
Fossil  shells  in  some  modern  deposits  have  been  scarcely  altered 
in  the  course  of  centuries,  having  simply  lost  a  part  of  their 
animal  matter.  But  in  other  cases  the  shell  has  disappeared,  and 
left  an  impression  only  of  its^  exterior,  or  a  cast  of  its  interior 
form,  or,  thirdly,  a  cast  of  the  shell  itself,  the  original  matter  of 
which  has  been  removed.  These  different  forms  of  fossilization 
may  easily  be  understood  if  we  examine  the  mud  recently  thrown 
out  from  a  pond  or  canal  in  which  there  are  shells.  If  the  mud 
be  argillaceous,  it  acquires  consistency  on  drying,  and  on  break- 
ing open  a  portion  of  it  we  find  that  each  shell  has  left  impres- 
sions of  its  external  form.  If  we  then  remove  the  shell  itself, 
we  find  within  a  solid  nucleus  of  clay,  having  the  form  of  the 
interior  of  the  shell.  This  form  is  often  very  different  from  that 
of  the  outer  shell.  Thus  a  cast  such  as  a,  Fig.  54.,  commonly 
called  a  fossil  screw,  would  never  be  suspected  by  an  inexperi- 
Fig.  54.  Fig.  55. 


Phasianella  Heddingtonensis, 
and  cast  of  the  same.    Coral  Rag. 


Trochus  Anglicus,  and 
cast.    Lias. 


enced  conchologist  to  be  the  internal  shape  of  the  fossil  univalve, 
b,  Fig.  54.  Nor  should  we  have  imagined  at  first  sight  that  the 
shell  a  and  the  cast  &,  Fig.  55,  were  different  parts  of  the  same 
fossil.  The  reader  will  observe,  in  the  last-mentioned  figure 
(&,  Fig.  55.),  that  an  empty  space  shaded  dark,  which  the  shell 
itself  once  occupied,  now  intervenes  between  the  enveloping 
stone  and  the  cast  of  the  smooth  interior  of  the  whorls.  In  such 


56       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Mineralization  of  Organic  Remains. 


cases  the  shell  has  been  dissolved  and  the  component  particles 
removed  by  water  percolating  the  rock.  If  the  nucleus  were 
taken  out,  a  hollow  mould  would  remain,  on  which  the  external 
form  of  the  shell  with  its  tubercles  and  striae,  as  seen  in  a,  Fig. 
55.,  would  be  seen  embossed.  Now  if  the  space  alluded  to 
between  the  nucleus  and  the  impression,  instead  of  being  left 
empty,  has  been  filled  up  with  calcareous  spar,  pyrites,  or  other 
mineral,  we  then  obtain  from  the  mould  an  exact  cast  both  of  the 
external  and  internal  form  of  the  original  shell.  In  this  manner 
silicified  casts  of  shells  have  been  formed ;  and  if  the  mud  or 
sand  of  the  nucleus  happen  to  be  incoherent,  or  soluble  in  acid, 
we  can  then  procure  in  flint  an  empty  shell  which  is  the  exact 
counterpart  of  the  original.  This  cast  may  be  compared  to  a 
bronze  statue,  representing  merely  the  superficial  form,  and  not 
the  internal  organization ;  but  there  is  another  description  of  petri- 
faction by  no  means  uncommon,  and  of  a  much  more  wonder- 
ful kind,  which  may  be  compared  to  certain  anatomical  models 
in  wax,  where  not  only  the  outward  forms  and  features,  but  the 
nerves,  blood-vessels,  and  other  internal  organs  are  also  shown. 
Thus  we  find  corals,  originally  calcareous,  in  which  not  only  the 
general  shape,  but  also  the  minute  and  complicated  internal 
organization  are  retained  in  flint. 

Such  a  process  of  petrifaction  is  still  more  remarkably  exhi- 
bited in  fossil  wood,  in  which  we  often  perceive  not  only  the 
rings  of  annual  growth,  but  all  the  minute  vessels  and  medullary 
rays.  Many  of  the  minute  pores  and  fibres  of  plants,  and  even 
those  spiral  vessels  which  in  the  living  vegetable  can  only  be 
discovered  by  the  microscope,  are  preserved.  Among  many 
instances  I  may  mention  a  fossil  tree,  seventy -two  feet  in  length, 
found  at  Gosforth  near  Newcastle,  in  sandstone  strata  associated 
with  coal.  But  cutting  a  transverse  slice  so  thin  as  to  transmit 
light,  and  magnifying  it  about  fifty-five  times,  the  texture  seen 
in  Fig.  56.  is  exhibited.  A  texture  equally  minute  and  compli- 
cated has  been  observed  in  the  wood  of  large  trunk*  of  fossil 
•p-  gg  trees  found  in  the  Craigleith  quarry  near 

Edinburgh,  where  the  stone  was  not  in 
the  slightest  degree  siliceous,  but  consisted 
chiefly  of  carbonate  of  lime,  with  oxide 
of  iron,  alumina,  and  carbon.  In  some 
examples  the  woody  fibre  is  partially  pre- 
served, but  it  has  entirely  vanished  from 
others. 

Texture  of  a  tree  from    the        ^  attemPtmg  tO  exP^in  the  prOCCSS  of 

coatetrata^magnifcd^^vi-  petrifaction  in  such  cases,  we  may  first 
tham-)  assume  that  strata  are  very  generally  per- 


PART  I.     CHAPTER  IV.  57 


Mineralization  of  Organic  Remains. 


mealed  by  water  charged  with  minute  portions  of  calcareous, 
siliceous,  and  other  earths  in  solution.  In  what  manner  they 
become  so  impregnated  will,  be  afterwards  considered.  If  an 
organic  substance  is  exposed  in  the  open  air  to  the  action  of  the 
sun  and  rain,  it  will  in  time  putrefy,  or  be  dissolved  into  its  com- 
ponent elements,  which  consist  chiefly  of  oxygen,  hydrogen,  and 
carbon.  These  will  readily  be  absorbed  by  the  atmosphere  or  be 
washed  away  by  rain,  so  that  all  vestiges  of  the  dead  animal  or 
plant  disappear.  But  if  the  same  substances  be  submerged  in 
water,  they  decompose  more  gradually  ;  and  if  buried  in  earth, 
still  more  slowly,  as  in  the  familiar  example  of  wooden  piles  or 
other  buried  timber.  Now,  if  as  fast  as  each  particle  is  set  free 
by  putrefaction  in  a  fluid  or  gaseous  state,  a  particle  equally 
minute  of  carbonate  of  lime,  flint,  or  other  mineral,  is  precipi- 
tated, we  may  imagine  this  inorganic  matter  to  take  the  place 
just  before  left  unoccupied  by  the  organic  molecule.  In  this 
manner  a  cast  of  the  interior  of  certain  vessels  may  first  be 
taken,  and  afterwards  the  walls  of  the  same  may  decay  and 
suffer  a  like  transmutation.  Yet  when  the  whole  is  lapidified,  it 
may  not  form  one  homogeneous  mass  of  stone  or  metal.  Some 
of  the  original  ligneous,  osseous,  or  other  organic  elements  may 
remain  mingled  in  certain  parts,  or  the  lapidifying  mineral  itself 
may  be  so  crystallized  in  different  parts  as  to  reflect  light  differ- 
ently, and  thus  the  texture  of  the  original  body  may  be  faithfully 
exhibited. 

But  the  student  will  ask  whether,  on  chemical  principles,  we 
have  reason  to  expect  that  mineral  matter  will  be  thrown  down 
precisely  in  those  spots  where  organic  decomposition  is  in  pro- 
gress ?  The  following  curious  experiments  may  serve  to  illus- 
trate this  point.  Professor  Goppert  of  Breslau  attempted  recently 
to  imitate  the  natural  process  of  petrifaction.  For  this  purpose 
he  steeped  a  variety  of  animal  and  vegetable  substances  in  waters, 
some  holding  siliceous,  others  calcareous,  others  metallic  matter 
in  solution.  He  found  that  in  the  period  of  a  few  weeks,  or  even 
days,  the  organic  bodies  thus  immersed  were  mineralized  to  a 
certain  extent.  Thus,  for  example,  thin  vertical  slices  of  deal, 
taken  from  the  Scotch  fir  (Pinus  Sylvestris),  were  immersed  in 
a  moderately  strong  solution  of  sulphate  of  iron.  When  they 
had  been  thoroughly  soaked  in  the  liquid  for  several  days,  they 
were  dried  and  exposed  to  a  red-heat  until  the  vegetable  matter 
was  burnt  up  and  nothing  remained  but  an  oxide  of  iron,  which 
was  found  to  have  taken  the  form  of  the  deal  so  exactly  that 
even  the  dotted  vessels  peculiar  to  this  family  of  plants,  and 
resembling  those  in  Fig.  56.,  were  distinctly  visible  under  the 
microscope. 


58       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Mineralization  of  Organic  Remains. 


Another  accidental  experiment  has  been  recorded  by  Mr.  Pepys 
in  the  Geological  Transactions.*  An  earthen  pitcher  containing 
several  quarts  of  sulphate  of  iron  had  remained  undisturbed  and 
unnoticed  for  about  a  twelvemonth  in  the  laboratory.  At  the  end 
of  this  time  when  the  liquor  was  examined  an  oily  appearance 
was  observed  on  the  surface,  and  a  yellowish  powder,  which 
proved  to  be  sulphur,  together  with  a  quantity  of  small  hairs. 
At  the  bottom  were  discovered  the  bones  of  several  mice  in  a 
sediment  consisting  of  small  grains  of  pyrites,  others  of  sulphur, 
others  of  crystallized  green  sulphate  of  iron,  and  a  black  muddy 
oxide  of  iron.  It  was  evident  that  some  mice  had  accidentally 
been  drowned  in  the  fluid,  and  by  the  mutual  action  of  the  animal 
matter  and  the  sulphate  of  iron  on  each  other,  the  metallic  sul- 
phate had  been  deprived  of  its  oxygen ;  hence  the  pyrites  and  the 
other  compounds  were  thrown  down.  Although  the  mice  were 
not  fossilized,  or  turned  into  pyrites,  the  phenomenon  shows  how 
mineral  waters,  charged  with  sulphate  of  iron,  may  be  deoxy- 
dated  on  coming  in  contact  with  animal  matter  undergoing  putre- 
faction, so  that  atom  after  atom  of  pyrites  may  be  precipitated, 
and  ready,  under  favourable  circumstances,  to  replace  the  oxy- 
gen, hydrogen,  and  carbon  into  which  the  original  body  would 
be  resolved. 

The  late  Dr.  Turner  observes,  that  when  mineral  matter  is  in 
a  "  nascent  state,"  that  is  to  say,  just  liberated  from  a  previous 
state  of  chemical  combination,  it  is  most  ready  to  unite  with 
other  matter,  and  form  a  new  chemical  compound.  Probably 
the  particles  or  atoms  just  set  free  are  of  extreme  minuteness, 
and  therefore  move  more  freely,  and  are  more  ready  to  obey  any 
impulse  of  chemical  affinity.  Whatever  be  the  cause,  it  clearly 
follows,  as  before  stated,  that  where  organic  matter  newly  imbed- 
ded in  sediment  is  decomposing,  there  will  chemical  changes 
take  place  most  actively. 

An  analysis  was  lately  made  of  the  water  which  was  flowing 
off  from  the  rich  mud  deposited  by  the  Hooghly  river  in  the 
Delta  of  the  Ganges  after  the  annual  inundation.  This  water 
was  found  to  be  highly  charged  with  carbonic  acid  gas  holding 
lime  in  solution.f  Now  if  newly  deposited  mud  is  thus  proved 
to  be  permeated  by  mineral  matter  in  a  state  of  solution,  it  is  not 
difficult  to  perceive  that  decomposing  organic  bodies,  naturally 
imbedded  in  sediment,  may  as  readily  become  petrified  as  the 
substances  artificially  immersed  by  Professor  Goppert  in  various 
fluid  mixtures. 


*  Vol.  i.  p.  399,  first  series. 

t  Piddinglon,  Asiat.  Researches,  vol.  xviii.  p.  226. 


PART  I.     CHAPTER  IV.  59 


Flint  of  Silicifiecl  Fossils,  whence  derived. 


It  is  well  known  that  the  water  of  springs,  or  that  which  is 
continually  percolating  the  earth's  crust,  is  rarely  free  from  a 
slight  admixture  either  of  iron,  carbonate  of  lime,  sulphur,  flint, 
potash,  or  some  other  earthy,  alkaline,  or  metallic  ingredient. 
Hot  springs  in  particular  are  copiously  charged  with  one  or  more 
of  these  elements ;  and  it  is  only  in  their  waters  that  silex  is 
found  in  abundance.  In  certain  cases,  therefore,  especially  in 
volcanic  regions,  we  may  imagine  the  flint  of  silicified  wood  and 
corals  to  have  been  supplied  by  the  waters  of  thermal  springs. 
In  other  instances,  as  in  tripoli  and  chalk-flint,  it  may  have  been 
derived  in  great  part,  if  not  wholly,  from  the  decomposition  of 
infusoria,  sponges,  and  other  bodies.  But  even  if  this  be  granted, 
we  have  still  to  inquire  whence  a  lake  or  the  ocean  can  be  con- 
stantly replenished  with  the  siliceous  matter  so  abundantly  with- 
drawn from  it  by  the  secretions  of  these  zoophytes. 

In  regard  to  carbonate  of  lime  there  is  no  difficulty,  because 
not  only  are  calcareous  springs  very  numerous,  but  even  rain- 
water has  the  power  of  dissolving  a  minute  portion  of  the  calca- 
reous rocks  over  which  it  flows.  Hence  marine  corals  and  mbl- 
lusca  may  be  provided  by  rivers  with  the  materials  of  their  shells 
and  solid  supports.  But  pure  silex,  even  when  reduced  to  the 
finest  powder  and  boiled,  is  insoluble  in  water.  Nevertheless 
Dr.  Turner  has  well  explained,  in  an  essay  on  the  chemistry  of 
geology,*  how  the  decomposition  of  felspar  may  be  a  source  of 
silex  in  solution,  as  widely  spread  as  are  the  felspathic  rocks 
which  form  so  large  a  proportion  of  the  volcanic,  plutonic,  and 
metamorphic  rocks,  and  are  therefore  universal,  occurring  some- 
where in  the  course  of  every  large  river.'' 

The  siliceous  earth,  which  constitutes  more  than  half  the  bulk 
of  felspar,  is  intimately  combined  with  alumine,  potash,  and 
some  other  elements.  The  alkaline  matter  of  the  felspar  has  a 
chemical  affinity  for  water,  as  also  for  the  carbonic  acid  which 
is  more  or  less  contained  in  the  waters  of  most  springs.  The 
water  therefore  carries  away  alkaline  matter,  and  leaves  behind 
a  clay  consisting  of  alumine  and  flint.  But  this  residue  of  the 
decomposed  mineral,  which  in  its  purest  state  is  called  porcelain- 
clay,  is  found  to  contain  only  a  small  portion  of  the  silica 
which  existed  in  the  original  felspar.  The  other  part  therefore 
must  have  been  dissolved  and  removed;  and  this  can  be 
accounted  for  in  two  ways,  first,  because  silex  when  combined 
with  an  alkali  is  soluble  in  water ;  secondly,  because  silex  in 
what  is  technically  called  its  nascent  state  is  also  soluble  in 
water.  Hence  an  endless  supply  of  silica  is  afforded  to  the 
waters  of  the  sea. 

*  Jam.  Ed.  New  Phil.  Journ.  No.  30.  p.  246. 


60       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Process  of  Petrifaction. 


The  disintegration  of  mica  also,  another  mineral  which  enters 
largely  into  the  composition  of  granite  and  various  sandstones, 
may  yield  silex  which  may  be  dissolved  in  water,  for  nearly 
half  of  this  mineral  consists  of  silica,  combined  with  alumine, 
potash,  and  about  a  tenth  part  of  iron.  The  oxidation  of  this 
iron  in  the  air  is  the  principal  cause  of  the  waste  of  mica. 

We  have  still,  however,  much  to  learn  before  the  conversion 
of  fossil  bodies  into  stone  is  fully  understood.  Some  phenomena 
seem  to  imply  that  the  mineralization  must  proceed  with  con- 
siderable rapidity,  for  stems  of  a  soft  and  succulent  character, 
and  of  a  most  perishable  nature,  are  preserved  in  flint ;  and 
there  are  instances  of  the  complete  silicification  of  the  young 
leaves  of  a  palm-tree  when  just  about  to  shoot  forth,  and  in  that 
state  which  in  the  West  Indies  is  called  the  cabbage  of  the 
palm.*  It  may,  however,  be  questioned  whether  in  such  cases 
there  may  not  have  been  some  anti-septic  quality  in  the  water 
which  retarded  putrefaction,  so  that  the  soft  parts  of  the  fturied 
substance  may  have  remained  for  a  long  time  without  disintegra- 
tion, like  the  flesh  of  bodies  imbedded  in  peat. 

Mr.  Stokes  has  pointed  out  examples  of  petrifactions  in  which 
the  more  perishable,  and  others  where  the  more  durable  portions 
of  wood  are  preserved.  These  variations,  he  suggests,  must 
doubtless  depend  on  the  time  when  the  lapidifying  mineral  was 
introduced.  Thus  in  certain  silicified  stems  of  palm-trees  the 
cellular  tissue,  that  most  destructible  part,  is  in  good  condition, 
all  signs  of  the  hard  woody  fibre  having  disappeared,  and  the 
spaces  once  occupied  by  it  being  hollow  or  filled  with  agate. 
Here  petrifaction  mustfnave  commenced  soon  after  the  wood  was 
exposed  to  the  action  of  moisture,  and  the  supply  of  mineral 
matter  must  have  failed,  or  the  water  have  become  too  much 
diluted  before  the  woody  fibre  decayed.  But  when  the  latter  is 
alone  discoverable,  we  must  then  suppose  that  an  interval  of 
time  elapsed  before  the  commencement  of  lapidification,  during 
which  the  cellular  tissue  was  obliterated.  When  both  structures, 
namely  the  cellular  and  the  woody  fibre,  are  preserved,  the  pro- 
cess must  have  commenced  at  an  early  period,  and  continued 
without  interruption  till  it  was  completed  throughout.! 

*  Stokes,  Geol.  Trans,  vol.  v.  p.  212.  second  series.  t  Ibid. 


PART  I.     CHAPTER  V.  01 


Land  has  been  raised,  not  the  sea  lowered. 


CHAPTER  V. 

ELEVATION   OF   STRATA   ABOVE   THE   SEA  — HORIZONTAL  AND  INCLINED 
STRATIFICATION. 

Why  the  elevated  position  of  marine  strata  should  be  referred  to  the  rising  up 
of  the  land,  not  to  the  going  down  of  the  sea — Upheaval  of  extensive  masses  of 
horizontal  strata — Inclined  and  vertical  stratification — Anticlinal  and  synclinal 
lines — Examples  of  bent  strata  in  east  of  Scotland — Theory  of  folding  by  lateral 
movement — Dip  and  strike — Structure  of  the  Jura — Rocks  broken  by  flexure — 
Inverted  position  of  disturbed  strata — Unconfbrmable  stratification — Fractures 
of  strata — Polished  surfaces — Faults — Appearance  of  repeated  alternations  pro- 
duced by  them — Origin  of  great  faults. 

LAND  has  been  raised,  not  the  sea  lowered. — It  has  been 
already  stated  that  the  aqueous  rocks  contaiaing  marine  fossils 
extend  over  wide  continental  tracts,  and  are  seen  in  mountain 
chains,  rising  to  great  heights  above  the  level  of  the  sea.  Hence 
it  follows  that  what  is  now  dry  land  was  once  under  water.  But 
if  we  admit  this  conclusion,  we  must  imagine  either  that  there  has 
been  a  general  lowering  of  the  waters  of  the  ocean,  or  that  the 
solid  rocks,  once  covered  by  water,  have  been  raised  up  bodily 
out  of  the  sea,  and  have  thus  become  dry  land.  The  earlier 
geologists,  finding  themselves  reduced  to  this  alternative,  em- 
braced the  former  opinion,  assuming  that  the  ocean  was  origin- 
ally universal,  and  had  gradually  sunk  down  to  its  actual  level, 
so  that  the  present  islands  and  continents  were  left  dry.  It 
seemed  to  them  far  easier  to  conceive  that  the  water  had  gone 
down,  than  that  solid  land  had  risen  upwards  into  its  present 
position.  It  was,  however,  impossible  to  invent  any  satisfactory 
hypothesis  to  explain  the  disappearance  of  so  enormous  a  body 
of  water  throughout  the  globe,  it  being  necessary  to  infer  that 
the  ocean  had  once  stood  at  whatever  height  marine  shells  were 
detected.  It  moreover  appeared  clear,  as  the  science  of  Geology 
advanced,  that  certain  spaces  on  the  globe  had  been  alternately 
sea,  then  land,  then  estuary,  then  sea  again,  and,  lastly,  once 
more  habitable  land,  having  remained  in  each  of  these  states  for 
considerable  periods.  In  order  to  account  for  such  phenomena, 
without  admitting  any  movement  of  the  land  itself,  we  are 
required  to  imagine  several  retreats  and  returns  of  the  ocean  ; 
and  even  then  our  theory  applies  merely  to  cases  where  the 
marine  strata  composing  the  dry  land  are  horizontal,  leaving 


02       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Land  has  been  raised,  not  the  Sea  lowered. 

unexplained  those  more  common  instances  where  strata  are 
inclined,  curved,  or  placed  on  their  edges,  and  evidently  not  in 
the  position  in  which  they  were  first  deposited. 

Geologists,  therefore,  were  at  last  compelled  to  have  recourse 
to  the  other  alternative,  namely,  the  doctrine  that  the  solid  land 
has  been  repeatedly  moved  upwards  or  downwards,  so  as  per- 
manently to  change  its  position  relatively  to  the  sea.  There  are 
several  distinct  grounds  for  preferring  this  conclusion.  First,  it 
will  account  equally  for  the  position  of  those  elevated  masses  of 
marine  origin  in  which  the  stratification  remains  horizontal,  and 
for  those  in  which  the  strata  are  disturbed,  broken,  inclined,  or 
vertical.  Secondly,  it  is  consistent  with  human  experience  that 
land  should  rise  gradually  in  some  places  and  be  depressed  in 
others.  Such  changes  have  actually  occurred  in  our  own  days, 
and  are  now  in  progress,  being  accompanied  in  some  cases  by 
violent  convulsions,  while  in  others  they  proceed  insensibly,  or 
are  only  ascertainable  by  the  most  careful  scientific  observations. 

On  the  other  hand,  there  is  no  evidence  from  human  experi- 
ence of  a  lowering  of  the  sea's  level  in  any  region,  and  the  wa- 
ters of  the  ocean  cannot  sink  in  one  place  without  their_  level 
being  depressed  every  where  throughout  the  globe. 

These  preliminary  remarks  will  prepare  the  reader  to  under- 
stand the  great  theoretical  interest  attached  to  all  facts  connected 
with  the  position  of  strata,  whether  horizontal  or  inclined,  curved 
or  vertical. 

Now  the  first  and  most  simple  appearance  is  where  strata  of 
marine  origin  occur  above  the  level  of  the  sea  in  horizontal  posi- 
tion. Such  are  the  strata  which  we  meet  with  in  the  south  of  Sicily, 
filled  with  shells  of  the  same  species  as  now  live  in  the  Mediter- 
ranean. Some  of  these  rocks  rise  to  the  height  of  2000  feet  and 
more  above  the  sea.  Other  mountain  masses  might  be  men- 
tioned with  horizontal  strata  of  high  antiquity  which  contain  fos- 
sils wholly  dissimilar  in  form  to  any  now  known  to  exist,  as  in 
the  south  of  Sweden,  near  Lake  Wener,  where  the  beds  of  a 
deposit,  called  Transition  or  Silurian  by  geologists,  occur  in  as 
level  a  position  as  if  they  had  recently  formed  part  of  the  delta 
of  a  great  river,  and  been  left  dry  on  the  retiring  of  the  annual 
floods.  Aqueous  rocks  of  about  the  same  age  extend  over  the 
lake-district  of  North  America,  exhibiting  in  like  manner  a  strati- 
fication nearly  undisturbed.  The  Table  Mountain  at  the  Cape 
of  Good  Hope  is  another  example  of  highly  elevated  and  per- 
fectly horizontal  strata,  no  less  than  3500  feet  in  thickness,  and 
consisting  of  sandstone  of  very  ancient  data. 

Instead  of  imagining  that  such  fossiliferous  rocks  were  always 
at  their  present  level,  and  that  the  sea  was  once  high  enough  to 


PART  I.    CHAPTER  V. 


Elevation  of  Land. 


cover  them,  we  suppose  them  to  have  constituted  the  ancient  bed 
of  the  ocean,  and  that  they  were  gradually  uplifted  to  their  pre- 
sent height.  This  idea,  however  startling  it  may  at  first  appear, 
is  quite  in  accordance,  as  above  stated,  with  the  analogy  of 
changes  now  going  on  in  certain  regions  of  the  globe.  Thus  in 
parts  of  Sweden,  for  example,  and  the  shores  and  islands  of  the 
Gulf  of  Bothnia,  proofs  have  been  obtained  that  the  land  is  ex- 
periencing, and  has  experienced  for  centuries,  a  slow  upheaving 
movement.  Play  fair  argued  in  favour  of  this  opinion  in  1802, 
and  in  1807  Von  Buch,  after  his  travels  in  Scandinavia,  an- 
nounced his  conviction  that  a  rising  of  the  land  was  in  progress. 
Celsius  and  other  Swedish  writers  had,  a  century  before,  declared 
their  belief  that  a  gradual  change  had  for  ages  been  taking  place 
in  the  relative  level  of  land  and  sea.  They  attributed  the  change 
to  a  fall  of  the  waters  both  of  the  ocean  and  the  Baltic ;  but  this 
theory  has  now  been  refuted  by  abundant  evidence ;  for  the  alter- 
ation of  relative  level  has  neither  been  universal  nor  every  where 
uniform  in  quantity,  but  has  amounted  in  some  regions  to  seve- 
ral feet  in  a  century,  in  others  to  a  few  inches,  while  in  the 
southernmost  part  of  Sweden,  or  the  province  of  Scania,  there 
has  been  actually  a  loss  instead  of  a  gain  of  land,  buildings  hav- 
ing gradually  sunk  below  the  level  of  the  sea.* 

It  appears  from  the  observations  of  Mr.  Darwin  and  others  that 
very  extensive  regions  of  the  continent  of  South  America  have 
been  undergoing  slow  and  gradual  upheaval,  by  which  the  level 
plains  of  Patagonia,  covered  with  recent  marine  shells,  and  the 
Pampas  of  Buenos  Ayres  have  been  formed. f  On  the  other 
hand  the  gradual  sinking  of  part  of  the  west  coast  of  Greenland, 
for  the  space  of  more  than  600  miles  from  north  to  south,  during 
the  last  four  centuries,  has  been  established  by  the  observations 
of  a  Danish  naturalist,  Dr.  Pingel.  And  while  these  proofs  of 
continental  elevation  and  subsidence  by  slow  and  insensible  move- 
ments have  been  recently  brought  to  light,  the  evidence  is  daily 
strengthened  of  continued  changes  of  level  effected  by  violent 
convulsions  in  countries  where  earthquakes  are  frequent.  Here 
the  rocks  are  rent  from  time  to  time,  and  heaved  up  or  thrown 
down  several  feet  at  once,  and  disturbed  in  such  a  manner,  that 
the  original  position  of  strata  may,  in  the  course  of  centuries,  be 
modified  to  any  amount. 

*  In  the  first  three  editions  of  my  Principles  of  Geology,  I  expressed  many 
doubts  as  to  the  validity  of  the  alleged  proofs  of  a  gradual  rise  of  land  in  Sweden  ; 
but  after  visiting  that  country  in  1834 1  retracted  these  objections,  and  published 
a  detailed  statement  of  the  observations  which  led  me  to  alter  my  opinion  in  the 
Phil.  Trans.  1835,  Part  I.  See  also  Principles,  5th  edition,  [1st.  Am.  Edit.]  book 
ii.  chap.  17. 

t  See  his  Journal  in  Voyage  of  the  Beagle. 


04 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Rising  and  Sinking  of  Land Vertical,  Inclined,  and  Curved  Stratification. 

It  has  also  been  shown  by  Mr.  Darwin,  that,  in  those  seas 
where  circular  coral  islands  abound,  there  is  a  slow  and  con- 
tinued sinking  of  the  submarine  mountains  on  which  these  masses 
of  coral  are  based  ;  while  in  other  areas  of  the  South  Sea,  where 
coral  is  found  above  the  sea  level,  arid  in  inland  situations,  and 
where  there  are  no  circular  or  barrier  reefs,  the  land  is  on  the 
rise.* 

It  would  require  a  volume  to  explain  to  the  reader  the  various 
facts  and  phenomena  which  confirm  the  reality  of  these  move- 
ments of  land,  whether  of  elevation  or  depression,  whether  ac- 
companied by  earthquakes  or  accomplished  slowly  and  without 
local  disturbance.  Having  treated  fully  of  these  subjects,  in  the 
Principles  of  Geology,  I  must  assume,  in  the  present  work,  that 
such  changes  are  part  of  the  actual  course  of  nature ;  and  when 
admitted,  they  will  be  found  to  afford  a  key  to  the  interpretation 
of  a  variety  of  geological  appearances,  such  as  the  elevation  of 
horizontal  or  disturbed  marine  strata,  the  superposition  of  fresh- 
water to  marine  deposits,  and  many  other  phenomena,  afterwards 
to  be  described.  It  will  also  appear,  in  the  second  part  of  this 
volume,  how  much  light  the  doctrine  of  a  continued  subsidence 
of  land  may  throw  on  the  manner  in  which  a  series  of  strata 
formed  in  shallow  water  may  have  accumulated  to  a  great  thick- 
ness. The  excavation  of  valleys  also,  and  other  effects  of  denu- 
dation, of  which  I  shall  presently  treat,  can  alone  be  understood 
when  we  duly  appreciate  the  proofs  now  on  record  of  the  pro- 
longed rising  and  sinking  of  land  throughout  wide  areas. 

Inclined  stratification. — The  most  unequivocal  evidence  of  a 
change  in  the  original  position  of  strata  is  afforded  by  their  stand- 
ing up  perpendicularly  on  their  edges,  which  is  by  no  means  a 
rare  phenomenon,  especially  in  mountainous  countries.  Thus 
we  find  in  Scotland,  on  the  southern  skirts  of  the  Grampians, 
beds  of  puddingstone  alternating  with  thin  layers  of  fine  sand, 

all  placed  vertically  to  the  horizon. 
When  Saussure  first  observed  cer- 
tain conglomerates  in  a  similar  po- 
sition in  the  Swiss  Alps,  he  re- 
marked that  the  pebbles,  being  for 
the  most  part  of  an  oval  shape,  had 
their  longer  axes  parallel  to  the 
planes  of  stratification,  (See  fig. 
57.)  From  this  he  inferred  that 
such. strata  must,  at  first,  have  been 


Fig.  57. 


Vertical  conglomerate  and  sandstone. 


*  Proceedings  of  Geol.  Soc.  No.  51.  p.  552.,  and  his  Journal  in  Voyage  of  the 
Beagle,  vol.  iii.  p.  557. 


PART  I.     CHAPTER  V. 


65 


Curved  Strata. 


horizontal,  each  oval  pebble  having  originally  settled  at  the  bot- 
tom of  the  water,  with  its  longer  side  parallel  to  the  horizon,  for 
the  same  reason  that  an  egg  will  not  stand  on  either  end  if 
unsupported.  Some  few,  indeed,  of  the  rounded  stones  in  a  con- 
glomerate may  afford  exceptions 
to  the  above  rule,  for  the  same 
reason  that  we  see  on  a  shingle 
beach  an  occasional  oval  or  flat- 
sided  pebble  resting  on  its  end 
or  edge.  For  some  pebbles  hav- 
ing been  forced  along  the  bottom 
and  against  each  other,  may 
have  settled  in  this  position. 

Vertical  strata,  when  they  can 
be  traced  continuously  upwards 
or  downwards  for  some  depth, 
are  almost  invariably  seen  to  be 
parts  of  great  curves,  which  may 
have  a  diameter  of  a  few  yards, 
or  of  several  miles.    I  shall  first 
describe  two  curves  of  consider- 
1 1>  able  regularity,  which  occur  in 
.2 1  Forfarshire,   extending   over    a 
-  country  twenty  miles  in  breadth, 
from  the  foot  of  the  Grampians 
to  the  sea  near  Arbroath. 

The  mass  of  strata  here  shown 
may  be  nearly  2000  feet  in 
thickness,  consisting  of  red  and 
white  sandstone,  and  various  co- 
loured shales,  the  beds  being  dis- 
tinguishable into  four  principal 
groups,  namely,  No.  1.  red  marl 
or  shale ;  No.  2.  red  sandstone, 
used  for  building;  No.  3.  con- 
glomerate; and  No.  4.,  grey 
paving-stone,  and  tile-stone,  with 
green  and  reddish  shale,  con- 
taining peculiar  organic  re- 
mains. A  glance  at  the  section 
will  show  that  each  of  the  for- 
mations 2,  3,  4,  are  repeated 
thrice  at  the  surface,  twice  with 
a  southerly  and  once  with  a 
northerly  inclination  or  dip,  and  the  beds  in  No.  1.,  which  arc 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Curved  Strata. 


nearly  horizontal,  are  still  brought  up  twice  by  a  slight  curva- 
ture to  the  surface,  once  on  each  side  of  A.  Beginning  at  the 
north-west  extremity,  the  tile-stones  and  conglomerates  No.  4. 
and  No.  3.  are  vertical,  and  they  generally  form  a  ridge  paral- 
lel to  the  southern  skirts  of  the  Grampians.  The  superior  strata 
Nos.  2.  and  1.  become  less  and  less  inclined  on  descending  to 
the  valley  of  Strathmore,  where  the  strata,  having  a  concave 
bend,  are  said  by  geologists  to  lie  in  a  "trough"  or  "basin." 
Through  the  centre  of  this  valley  runs  an  imaginary  line  A, 
called  technically  a  "  synclinal  line,"  where  the  beds  which  are 
tilted  in  opposite  directions,  may  be  supposed  to  meet.  It  is  most 
important  for  the  observer  to  mark  such  lines,  for  he  will  per- 
ceive by  the  diagram,  that  in  travelling  from  the  north  to  the 
centre  of  the  basin,  he  is  always  passing  from  older  to  newer 
beds ;  whereas  after  crossing  the  line  A,  and  pursuing  his  course 
in  the  same  southerly  direction,  he  is  continually  leaving  the 
newer,  and  advancing  upon  older  strata.  All  the  deposits  which 
he  had  before  examined  begin  then  to  recur  in  reversed  order, 
until  he  arrives  at  the  central  axis  of  the  Sidlaw  hills,  where  the 
strata  are  seen  to  form  an  arch  or  saddle,  having  an  anticlinal 
line  B,  in  the  centre.  On  passing  this  line,  and  continuing  to- 
wards the  S.  E.,  the  formations  4,  3,  and  2,  are  again  repeated, 
in  the  same  relative  order  of  superposition,  but  with  a  northerly 
dip.  At  Whiteness  (see  diagram)  it  will  be  seen  that  the  inclined 
strata  are  covered  by  a  newer  deposit,  a,  in  horizontal  beds. 
These  are  composed  of  red  conglomerate  and  sand,  and  are 
newer  than  any  of  the  groups,  1,  2,  3,  4,  before  described,  and 
rest  unconformably  upon  strata  of  the  sand-stone  group,  No.  2. 

Fig.  59. 


rved  strata  of  slate  near  St,  AWs  Head,  Berwickshire. 


An  example  of  curved  strata,  in  which  the  bends  or  convolu- 
tions of  the  rock  are  sharper  and  far  more  numerous  within  an 


PART  I.     CHAPTER  V. 


67 


Experiments  to  illustrate  curved  Strata. 


equal  space,  has  been  well  described  by  Sir  James  Hall.*  It 
occurs  near  St.  Abb's  Head,  on  the  east  coast  of  Scotland,  where 
the  rocks  consist  principally  of  a  bluish  slate,  having  frequently 
a  ripple-marked  surface.  The  undulations  of  the  beds  reach 
from  the  top  to  the  bottom  of  the  cliffs  from  200  to  300  feet  in 
height,  and  there  are  sixteen  distinct  bendings  in  the  course  of 
about  six  miles,  the  curvatures  being  alternately  concave  and 
convex  upwards. 

An  experiment  was  made  by  Sir  James  Hall,  with  a  view  of 
illustrating  the  manner  in  which  such  strata,  assuming  them  to 
have  been  originally  horizontal,  may  have  been  forced  into  their 
present  position.  A  set  of  layers  of  clay  were  placed  under  a 
weight,  and  their  opposite  ends  pressed  towards  each  other  with 
such  force  as  to  cause  them  to  approach  more  nearly  together. 
On  the  removal  of  the  weight,  the  layers  of  clay  were  found  to 
be  curved  and  folded,  so  as  to  bear  a  miniature  resemblance  to 
the  strata  in  the  cliffs.  We  must,  however,  bear  in  mind,  that 
in  the  natural  section  or  sea-cliff  we  only  see  the  foldings  imper- 
fectly, one  part  being  invisible  beneath  the  sea,  and  the  other,  or 
upper  portion,  being  supposed  to  have  been  carried  away  by  de- 
nudation, or  that  action  of  water  which  will  be  explained  in  the 
next  chapter.  The  dark  lines  in  the  accompanying  plan  (Fig. 

)f 


60.),  represent  what  is  actually  seen  of  the  strata  in  part  of 
Fig.  60. 


the 


line  of  cliff  alluded  to ;  the  fainter  lines,  that  portion  which  is 
concealed  beneath  the  sea  level,  as  also  that  which  is  supposed 
to  have  once  existed  above  the  present  surface. 

We  may  still  more  easily  illustrate  the  effects  which  a  lateral 
thrust  might  produce  on  flexible  strata,  by  placing  several  pieces 
of  differently  coloured  cloths  upon  a  table,  and  when  they  are 
spread  out  horizontally,  cover  them  with  a  book.  Then  apply 


*  Edin.  Trans,  vol.  vii.  pi.  3. 


68 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Origin  of  Curved  Strata Zigzag  Flexures. 

other  books  to  each  end,  and  force  them  towards  each  other. 

The  folding  of  the  cloths  will  exactly  imitate  those  of  the  bent 

strata.     (See  Fig.  61.) 

r  gj  To  inquire  whe- 

ther the  analogous 
flexures  in  strata 
have  really  been 
due  to  similar  side- 


way  movements,  or 
other  exertions  of 
force,  would  lead 
me  farther  into  the 
regions  of  specula- 
tion and  conjecture 
than  might  be  consistent  with  the  scope  of  this  elementary 
work.  When  the  volcanic  and  granitic  rocks  are  described, 
it  will  be  seen  that  some  of  them  have,  when  melted,  been  in- 
jected forcibly  into  fissures,  while  others,  already  in  a  solid 
state,  have  been  protruded  upwards  through  the  incumbent 
crust  of  the  earth,  by  which  a  great  displacement  of  flexible 
strata  must  have  been  caused.  It  also  appears  that  cavities  are 
sometimes  formed  in  the  interior  of  the  earth,  whether  by  the 
removal  of  matter  by  volcanic  action,  or  by  the  contraction  of 
argillaceous  rocks,  or  other  causes.  In  this  manner  pliable  beds 
sinking  down,  from  failure  of  support,  into  chasms  of  less  hori- 
zontal extent,  may  have  become  folded  and  compressed  laterally. 
Such  subsidences  have  been  witnessed  on  a  small  scale  in  the 
undermined  ground  immediately  over  coal-pits,  from  which  large 
quantities  of  coal  and  stone  had  been  extracted. 

Fig.  62. 


Zigzag  flexures  of  coal  near  Mons. 

Between  the  layers  of  shale,  accompanying  coal,  we  some- 


PART  I.  CHAPTER  V. 


Dip  and  Strike. 


times  see  the  leaves  of  fossil  ferns  spread  out  as  regularly  as 
dried  plants  between  sheets  of  paper  in  the  herbarium  of  a  bota- 
nist. These  fern  leaves,  or  fronds,  must  have  rested  horizontally 
on  soft  mud,  when  first  deposited.  If,  therefore,  tljey  and  the 
layers  of  shale  are  now  inclined,  or  standing  on  end,  it  is  obvi- 
ously the  effect  of  subsequent  derangement.  The  proof  becomes, 
if  possible,  still  more  striking  when  these  strata,  including  vege- 
table remains,  are  curved  again  and  again,  and  even  folded  into 
the  form  of  the  letter  Z,  so  that  the  same  continuous  layer  of  coal 
is  cut  through  several  times  in  the  same  perpendicular  shaft. 
Thus,  in  the  coal-field  near  Mons,  in  Belgium,  these  zigzag  bend- 
ings  are  repeated  four  or  five  times,  in  the  manner  represented  in 
Fig  62.,  the  black  lines  representing  seams  of  coal.* 

Dip  and  Strike. — In  the  above  remarks  several  technical 
terms  have  been  used,  such  as  dip,  the  unconformable  position 
of  strata,  and  the  anticlinal  and  synclinal  lines,  which,  as  well 
as  the  strike  of  the  beds,  I  shall  now  explain.  If  a  stratum  or 
bed  of  rock,  instead  of  being  quite  level,  be  inclined  to  one  side, 
it  is  said  to  dip;  the  point  of  the  compass  to  which  it  is  inclined 
is  called  the  point  of  dip,  and  the  degree  of  deviation  from  a 
Y'lff  gg  level  or  horizontal  line  is 

fl  °'  w  called  the  amount  of  dip,  or 

the  angle  of  dip.  Thus,  in 
the  annexed  diagram  (Fig. 
63.),  a  series  of  strata  are 
inclined,  and  they  dip  to  the 
north  at  an  angle  of  forty- 
five  degrees.  The  strike,  or  line  of  bearing,  is  the  prolongation 
or  extension  of  the  strata  in  a  direction  at  right  angles  to  the 
dip;  and  hence  it  is  sometimes  called  the  direction  of  the  strata. 
Thus,  in  the  above  instance  of  strata  dipping  to  the  north,  their 
strike  must  necessarily  be  east  and  west.  We  have  borrowed 
the  word  from  the  German  geologists,  streichen  signifying  to 
extend,  to  have  a  certain  direction.  Dip  and  strike  may  be  aptly 
illustrated  by  a  row  of  houses  running  east  and  west,  the  long 
ridge  of  the  roof  representing  the  strike  of  the  stratum  of  slates, 
which  dip  on  one  side  to  the  north,  and  on  the  other  to  the  south. 
A  stratum  which  is  horizontal,  or  quite  level  in  all  directions, 
has  neither  dip  nor  strike. 

It  is  always  important  for  the  geologist,  who  is  endeavouring 
to  comprehend  the  structure  of  a  country,  to  learn  how  the  beds 
dip  in  every  part  of  the  district ;  but  it  requires  some  practice  to 


Sea  Level 


*  See  plan  by  M.  Chevalier,  Burat's  D'Aubuisson,  torn.  ii.  p.  331. 


70       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Dip  and  Strike. 

avoid  being  occasionally  deceived,  both  as  to  the  point  of  dip  and 
the  amount  of  it. 

If  the  upper  surface  of  a  hard  stony  stratum  be  uncovered, 
whether  artificially  in  a  quarry,  or  by  the  waves  at  the  foot  of  a 
cliff,  it  is  easy  to  determine  towards  what  point  of  the  compass 
the  slope  is  steepest,  or  in  what  direction  water  would  flow,  if 
poured  upon  it.  This  is  the  true  dip.  Perfectly  horizontal  lines 
in  the  face  of  a  vertical  cliff  may  be  the  edges  of  highly  inclined 
strata,  if  the  observer  see  them  in  the  line  of  their  strike,  their 
dip  being  inwards  from  the  face  of  the  cliff.  If,  however,  we 
come  to  a  break  in  the  cliff,  which  exhibits  a  section  exactly  at 
right  angles  to  the  line  of  the  strike,  we  are  then  able  to  ascertain 
the  true  dip.  In  the  annexed  drawing  (Fig.  64.),  we  may  sup- 
Fig.  64. 


Apparent  horizontality  of  inclined  strata. 

pose  a  headland,  one  side  of  which  faces  to  the  north,  where  the 
beds  would  appear  perfectly  horizontal,  to  a  person  in  the  boat ; 
while  in  the  other  side  facing  the  west,  the  true  dip  would  be  seen 
by  the  person  on  shore  to  be  at  an  angle  of  40°.  If,  therefore, 
our  observations  are  confined  to  a  vertical  precipice  facing  in  one 
direction,  we  must  endeavour  to  find  a  ledge  or  portion  of  the 
plane  of  one  of  the  beds  projecting  beyond  the  others,  in  order 
to  ascertain  the  true  dip. 

It  is  rarely  important  to  determine  the  angle  of  inclination 
with  such  minuteness  as  to  require  the  aid  of  the  instrument 
called  a  clinometer.  We  may  measure  the  angle  within  a  few 
degrees  by  standing  exactly  opposite  to  a  cliff  where  the  true  dip 
is  exhibited,  holding  the  hands  immediately  before  the  eyes,  and 
placing  the  fingers  of  one  in  a  perpendicular,  and  of  the  other  in 
a  horizontal  position,  as  in  Fig.  65.  It  is  thus  easy  to  discover 
whether  the  lines  of  the  inclined  beds  bisect  the  angle  of  90°, 


PART  I.     CHAPTER  V. 


71 


Dip  and  Strike. 


Fig.  65.  formed  by  the  meeting  of  the  hands, 

so  as  to  give  an  angle  of  45°,  or 
whether  it  would  divide  the  space 
into  two  equal  or  unequal  portions. 
The  upper  dotted  line  may  express 
a  stratum  dipping  to  the  north ; 
but  should  the  beds  dip  precisely  to 
the  opposite  point  of  the  compass 
as  in  the  lower  dotted  line,  it  will 
be  seen  that  the  amount  of  incli- 
nation may  still  be  measured  by 
the  hands  with  equal  facility. 

It  has  been  already  seen,  in  de- 
scribing the  curved  strata  on  the 
east  coast  of  Scotland,  in  Forfarshire  and  Berwickshire,  that  a 
series  of  concave  and  convex  bendings  are  occasionally  repeated 
several  times.  These  usually  form  part  of  a  series  of  parallel 
waves  of  strata,  which  are  prolonged  in  the  same  direction 
throughout  a  considerable  extent  of  country.  Thus,  for  exam- 
ple, in  the  Swiss  Jura,  that  lofty  chain  of  mountains  has  been 
proved  to  consist  of  many  parallel  ridges,  with  intervening  lon- 
gitudinal valleys,  as  in  Fig.  66.,  the  ridges  being  formed  by 

Fig.  66. 


Section  illustrating  the  structure  of  the  Swiss  Jura. 

curved  fossiliferous  strata,  of  which  the  nature  and  dip  are  occa- 
sionally displayed  in  deep  transverse  gorges,  called  "  cluses," 
caused  by  fractures  at  right  angles  to  the  direction  of  the  chain.* 
Now  let  us  suppose  these  ridges  and  parallel  valleys  to  run  north 
and  south,  we  should  then  say  that  the  strike  of  the  beds  is 

*  See  M.  Thurmann's  work,  "  Essai  sur  les  Soulevemens  Jurassiques  du  Por- 
rentruy,  Paris,  1832,"  with  whom  I  examined  part  of  these  mountains  in  1835. 


72 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Anticlinal  and  Synclinal  Lines. 


Fig.  67. 


Fig.  68. 


north  and  south,  and  the 
dip  east  and  west.  A  line 
|  drawn  along  the  summit  of 
|  the  ridges  A,  B  would  be 
|  an  anticlinal  line,  and  one 
g  following  the  bottom  of  the 
I  adjoining  valleys  a  syn- 
•  clinal  line.  It  will  be  ob- 

Oroundplan  of  the  denuded  ridge  C,fg.  66.  Served   that  SOme  of  these 

ridges,  A,  B,  are  unbroken  on  the  summit,  whereas  one  of  them, 
C,  has  been  fractured  along  the  line  of  strike,  and  a  portion  of 
it  carried  away  by  denudation,  so  that  the  edges  of  the  beds  in 
the  formations  a,  &,  c  come  out  to  the  day,  or,  as  the  miners  say, 
crop  out,  on  the  sides  of  a  valley.  The  ground  plan  of  such  a 
denuded  ridge  as  C  may  be  expressed  by  the  diagram  Fig.  67., 
and  the  cross  section  of  the  same  by  Fig.  68.  The  line  D  E, 
Fig.  67.,  is  the  anticlinal  line,  on  each  side  of  which  the  dip  is 
in  opposite  directions,  as  expressed  by  the  arrows.  The  emer- 
gence of  strata  at  the  surface  is  called  by  miners  their  outcrop 
or  basset. 

If,  instead  of  being  folded  into  parallel  ridges,  the  beds  form  a 
boss  or  dome-shaped  protuberance,  and  if  we  suppose  the  sum- 
mit of  the  dome  carried  off,  the  ground  plan  would  exhibit  the 
edges  of  the  strata  forming  a  succession  of  circles,  or  ellipses, 
round  a  common  centre.  These  circles  are  the  lines  of  strike, 
and  the  dip  being  always  at  right  angles  is  inclined  in  the  course 
of  the  circuit  to  every  point  of  the  compass,  constituting  what  is 
termed  a  qua-qua-versal  dip — that  is,  turning  each  way. 

In  the  majority  of  cases,  an  anticlinal  axis  forms  a  ridge,  and 
a  synclinal  axis  a  valley,  as  in  A,  B,  Fig.  58.  p.  65. ;  but  there 
are  exceptions  to  this  rule,  the  beds 
sometimes  sloping  inwards  from  either 
side  of  a  mountain,  as  in  Fig.  69. 

On  following  the  anticlinal  line  of 
the  ridges  of  the  Jura,  before  men- 
tioned, A,  B,  C,  Fig.  66.,  we  often 
discover  longitudinal  fissures  along  the 
line  where  the  flexure  was  greatest. 
At  the  eastern  extremity  of  the  Pyrenees  a  curious  illustration 
of  an  analogous  phenomenon  may  be  seen  on  a  small  scale  (Fig. 
70.)  The  strata  there  laid  open,  in  the  sea-cliffs,  consist  of 
marl,  grit,  and  chert,  belonging  to  a  formation  of  the  age  of  the 
green-sand  of  English  geologists.  Some  of  the  bendings  are  so 
sharp,  that  fragments  of  the  slaty  chert — a  hard  flinty  rock — 
taken  from  the  points  where  they  form  an  angle  at  a,  might  be 


PART  I.    CHAPTER  V. 


73 


Bent  Strata  in  Pyrenees Reversed  dip  of  Strata. 


Fig, 


Fig.  71. 


Strata  of  chert,  grit,  and  marl,  near  St.  Jean  de  Luz. 

used  for  ridge-tiles  on  the  roof  of  a  house.  Although  this  chert 
is  now  brittle,  we  must  necessarily  suppose  that  it  was  flexible 
when  folded  into  this  shape  ;  nevertheless  it  must  have  had  some 
solidity,  for  precisely  at  the  angle  of  flexure  there  are  numerous 
cracks  filled  with  calcedony.  There  are  also  some  veins  of 
quartz,  &,  Fig.  70.,  traversing  the  same  formation  which  have 
filled  irregular  fissures,  probably  enfiltered  at  the  same  time  as 
the  calcedony  above  mentioned. 

Between  San  Caterina  and  Castrogiovanni,  in'  Sicily,  bent  and 
undulating  gypseous  marls  occur,  with  here  and  there  thin  beds 
of  solid  gypsum  interstratified.  Sometimes  these  solid  layers 
have  been  broken  into  detached  fragments,  still  preserving  their 
sharp  edges,  (g  g,  Fig.  71.),  while 
the  continuity  of  the  more  pliable  and 
ductile  marls,  m  m,  has  not  been  inter- 
rupted. 

I  shall  conclude  my  remarks  on  bent 
strata  by  stating,  that,  in  mountainous 
regions  like  the  Alps,  it  is  often  difficult 
for  an  experienced  geologist  to  deter- 
mine correctly  the  relative  age  of 

,     j      ,  J      .  .  n     D, 

beds  by  superposition,  so  often  have 
the  strata  been  folded  back  upon  themselves,  the  upper  parts  of 
the  curve  having  been  removed  by  denudation.  Thus,  if  we  met 
with  the  strata  seen  in  the  section  Fig.  72.,  we  should  naturally 
suppose  that  there  were  twelve  distinct  beds,  or  sets  of  beds,  No. 

1.  being  the  youngest,  and  No.  12. 
the  oldest  of  the  series.  But  this 
section  may,  perhaps,  exhibit  mere- 
ly six  beds,  which  have  been  fold- 
ed in  the  manner  seen  in  Fig.  73., 
so  that  each  of  them  are  twice  repeated,  the  position  of  one  half 
being  reversed,  and  part  of  No.  1.,  originally  the  uppermost, 
having  now  become  the  lowest  of  the  series.  These  phenomena 
are  often  observable  on  a  magnificent  scale  in  certain  regions  in 
Switzerland,  where  there  are  precipices  from  2000  to  3000  feet 
in  perpendicular  height.  In  the  Iselten  Alp,  in  the  valley  of  the 


g.  gypsum,    m.  marl. 


p. 


74 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Curved  Strata  in  the  Alps Unconformable  Stratification 

Fig.  73.  Lutschine,  between  Un- 

terseen  and  Grindelwald, 
curves  of  calcareous 
shale  are  seen  from  1000 
to  1500  feet  in  height, 
in  which  the  beds  some- 
times plunge  down  ver- 
tically for  a  depth  of 
1000  feet  and  more,  be- 
fore they  bend  round 
again.  There  are  many 

flexures  not  inferior  in  dimensions  in  the  Pyrenees,  as  those  near 

Gavarnie,  at  the  base  of  Mont  Perdu. 


Fig.  74. 


Curved  strata  of  the  Iselten  Alp. 

Unconformable  stratification. — Strata  are  said  to  be  uncon- 
formable,  when  one  series  is  so  placed  over  another,  that  the 
planes  of  the  superior  repose  on  the  edges  of  the  inferior.  In 
this  case  it  is  evident  that  a  period  had  elapsed  between  the  pro- 
duction of  the  two  sets  of  strata,  and  that,  during  this  interval, 
the  inferior  series  had  been  tilted  and  disturbed.  Afterwards  the 
upper  series  was  thrown  down  in  horizontal  strata  upon  it.  If 
these  superior  beds  are  also  inclined,  it  is  plain  that  the  lower 
strata  have  been  twice  displaced ;  first,  when  they  were  them- 
selves brought  into  an  inclined  position,  and  a  second  time  when 
the  superior  beds  were  thrown  out  of  the  horizontal  line. 

It  often  happens  that  in  the  interval  between  the  deposition  of 
two  sets  of  unconforrnable  strata,  the  inferior  rock  has  been  de- 
nuded, and  sometimes  drilled  by  perforating  shells.  Thus,  for 


PART  I.     CHAPTER  V. 


75 


Fissures  in  Strata. 


example,  at  Autreppe  and  Gusigny,  near  Mons,  beds  of  ancient 
stone  commonly  called  transition  limestone,  highly  inclined,  and 
often  bent,  are  covered  with  horizontal  strata  of  greenish  and 
whitish  marls  of  the  cretaceous  formation,  which  will  be  men- 
tioned in  a  future  chapter.  The  lowest  and  therefore  the  oldest 
bed  of  the  horizontal  series  is  usually  the  sand  and  conglome- 
rate, a,  in  which  are  rounded  fragments  of  stone,  from  an  inch 


Fig.  75. 


Junction  of  unconformable  strata  near  Mons,  in  Belgium. 

to  two  feet  in  diameter.  These  fragments  have  often  adhering 
shells  attached  to  them,  and  have  been  bored  by  perforating  mol- 
lusca.  The  solid  surface  of  the  inferior  limestone  has  also  been 
bored,  so  as  to  exhibit  cylindrical  and  pear-shaped  cavities,  as  at 
£,  the  work  of  saxicavous  mollusca ;  and  many  rents,  as  at  &, 
which  descend  several  feet  or  yards  into  the  limestone,  have  been 
filled  with  sand  and  shells,  similar  to  those  in  the  stratum  a. 

Fractures  of  the  strata. — Numerous  rents  may  often  be  seen 
in  rocks  which  appear  to  have  been  simply  broken,  the  separated 
parts  remaining  in  the  same  places ;  but  we  often  find  a  fissure, 
several  inches  or  yards  wide,  intervening  between  the  disunited 
portions.  These  fissures  are  usually  filled  with  fine  earth  and 
sand,  or  with  angular  fragments  of  stone,  evidently  derived  from 
the  fracture  of  the  contiguous  rocks. 

The  face  of  each  wall  of  the  fissure  is  often  beautifully  pol- 
ished, as  if  glazed,  striated,  or  scored  with  parallel  furrows  and 
ridges,  such  as  would  be  produced  by  the  continued  rubbing  to- 
gether of  surfaces  of  unequal  hardness.  Those  polished  surfaces 
are  called  by  miners  "  slicken-sides."  It  is  supposed  that  the 
lines  of  the  striee  indicate  the  direction  in  which  the  rocks  were 
moved.  During  one  of  the  late  minor  earthquakes  in  Chili,  the 
brick  walls  of  a  building  were  rent  vertically  in  several  places,  and 
made  to  vibrate  for  several  minutes  during  each  shock,  after 
which  they  remained  uninjured,  and  without  any  opening,  al- 
though the  line  of  each  crack  was  still  visible.  When  all  move- 
ment had  ceased,  there  were  seen  on  the  floor  of  the  house,  at 
the  bottom  of  each  rent,  small  heaps  of  fine  brickdust,  evidently 
produced  by  trituration. 

Faults. — It  is  not  uncommon  to  find  the  mass  of  rock,  on  one 


76 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Faults. 


side  of  a  fissure,  thrown  up  above  or  down  below  the  mass  with 
which  it  was  once  in  contact  on  the  other  side.  This  mode  of 
displacement  is  called  a  shift,  slip,  or  fault.  "  The  miner,"  says 
Play  fair,  describing  a  fault,  "  is  often  perplexed,  in  his  subterra- 
neous journey,  by  a  derangement  in  the  strata,  which  changes 
at  once  all  those  lines  and  bearings  which  had  hitherto  directed 
his  course.  When  his  mine  reaches  a  certain  plane,  which  is 
sometimes  perpendicular,  as  in  A  B,  Fig.  76.,  sometimes  oblique 

Fig.  76. 


B  D 

Faults.    A  B  perpendicular,  C  D  oblique  to  the  horizon. 

to  the  horizon  (as  in  C  D,  ibid.),  he  finds  the  beds  of  rock  broken 
asunder,  those  on  the  one  side  of  the  plane  having  chaftged  their 
place,  by  sliding  in  a  particular  direction  along  the  face  of  the 
others.  In  this  motion  they  have  sometimes  preserved  their  pa- 
rallelism, as  in  Fig.  76.,  so  that  the  strata  on  each  side  of  the 
faults  A  B,  C  D,  continue  parallel  to  one  another ;  in  other  cases, 
the  strata  on  each  side  are  inclined,  as  in  a,  &,  c,  d,  (Fig.  77.), 

Fig.  77. 


E  F,  fault  or  fissure  filled  with  rubbish,  on  each  side  of  which  the  shifted 
strata  are  not  parallel. 

though  their  identity  is  still  to  be  recognized  by  their  possessing 
the  same  thickness,  and  the  same  internal  characters."* 

We  sometimes  see  exact  counterparts  of  these  slips,  on  a  small 
scale,  in  pits  of  fine  loose  sand  and  gravel,  many  of  whrch  have 
doubtless  been  caused  by  the  drying  and  shrinking  of  argillace- 
ous and  other  beds,  slight  subsidences  having  taken  place  from 

*  Playfair,  Illust.  of  Hutt.  Theory,  $  42. 


PART  I.  CHAPTER  V. 


77 


Faults. 


failure  of  support.  Sometimes,  however,  even  these  small  slips 
may  have  been  produced  during  earthquakes ;  for  land  has  been 
moved,  and  its  level,  relatively  to  the  sea,  considerably  altered, 
within  the  period  when  much  of  the  alluvial  sand  and  gravel 
now  covering  the  surface  of  continents  was  deposited. 

I  have  already  stated  that  a  geologist  must  be  on  his  guard,  in 
a  region  of  disturbed  strata,  against  inferring  repeated  alterna- 
tions of  rocks,  when  in  fact,  the  same  strata,  once  continuous, 
have  been  bent  round  so  as  to  recur  in  the  same  section,  and 
with  the  same  dip.  A  similar  mistake  has  often  been  occasioned 
by  a  series  of  faults. 

If,  for  example,  the  dark  line  A  H  (Fig.  78.)  represent  the 


Fig.  78. 


Apparent  alternations  of  strata  caused  by  vertical  faults. 

surface  of  a  country  on  which  the  strata  a  b  c  frequently  crop 
out,  an  observer,  who  is  proceeding  from  H  to  A,  might  at  first 
imagine  that  at  every  step  he  was  approaching  new  strata, 
whereas  the  repetition  of  the  same  beds  has  been  caused  by  ver- 
tical faults,  or  downthrows.  Thus,  suppose  the  original  mass, 
A,  B,  C,  D,  to  have  been  a  set  of  uniformly  inclined  strata,  and 
that  the  different  masses  under  E  F,  F  G,  and  G  D,  sank  down 
successively,  so  as  to  leave  vacant  the  spaces  marked  in  the  dia- 
gram by  dotted  lines,  and  to  occupy  those  marked  by  the  conti- 
nuous fainter  lines,  then  let  denudation  take  place  along  the  line 
A  H,  so  that  the  protruding  and  triangular  masses  indicated  by 
the  fainter  lines  are  swept  away, — a  miner,  who  has  not  discov- 
ered the  faults,  finding  the  mass  a,  which  we  will  suppose  to  be 
a  bed  of  coal  four  times  repeated,  might  hope  to  find  four  beds, 
workable  to  an  indefinite  depth,  but  on  arriving  at  the  fault  G  he 
is  stopped  suddenly  in  his  workings,  upon  reaching  the  strata  of 
sandstone  c,  or  on  arriving  at  the  line  of  fault  F  he  comes  partly 
upon  the  shale  6,  and  partly  on  the  sandstone  c,  and  on  reach- 
ing E  he  is  again  stopped  by  a  wall  composed  of  the  rock  d. 


i 
78       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Grooved  surfaces  of  Faults. 


The  very  different  levels  at  which  the  separated  parts  of  the 
same  strata  are  found  on  the  different  sides  of  the  fissure,  in 
some  faults,  is  truly  astonishing.  One  of  the  most  celebrated  in 
England  is  that  called  the  "  ninety-fathom  dike,"  in  the  coal-field 
of  Newcastle.  This  name  has  been  given  to  it,  because  the  same 
beds  are  ninety  fathoms  lower  on  the  northern  than  they  are  on 
the  southern  side.  The  fissure  has  been  filled  by  a  body  of  sand, 
which  is  now  in  the  state  of  sandstone,  and  is  called  the  dike, 
which  is  sometimes  very  narrow,  but  in  other  places  more  than 
twenty  yards  wide.*  The  walls  of  the  fissure  are  scored  by 
grooves,  such  as  would  have  been  produced  if  the  broken  ends 
of  the  rock  had  been  rubbed  along  the  plane  of  the  fault-f In 
the  Tynedale  and  Craven  faults,  in  the  north  of  England,  the 
vertical  displacement  is  still  greater,  and  the  horizontal  extent  of 
the  movement  is  from  twenty  to  forty  miles.  Some  geologists 
consider  it  necessary  to  imagine  that  the  upward  or  downward 
movement  in  these  cases  was  accomplished  at  a  single  stroke, 
and  not  by  a  series  of  sudden  but  interrupted  movements.  This 
idea  appears  to  have  been  derived  from  a  notion  that  the  grooved 
walls  have  merely  been  rubbed  in  one  direction.  But  this  is  so 
far  from  being  a  constant  phenomenon  in  faults,  that  it  has  often 
been  objected  to  the  received  theory  respecting  slickensides,  that 
the  strisB  are  not  always  parallel,  but  often  curved  and  irregular. 
It  has,  moreover,  been  remarked,  that  not  only  the  walls  of  the 
fissure  or  fault,  but  its  earthy  contents,  sometimes  present  the 
same  polished  and  striated  faces.  Now  these  facts  seem  to  indi- 
cate partial  changes  in  the  direction  of  the  movement,  and  some 
slidings  subsequent  to  the  first  filling  up  of  the  fissure.  Suppose 
the  mass  of  rock  A,  B,  C,  to  overlie  an  extensive  chasm  d  e, 

formed  at  the  depth 
of  several  miles, 
whether  by  the  gra- 
dual contraction  in 
bulk  of  a  mass  of 
strata,  baked  by  a 
moderate  heat,  or  by 
the  subtraction  of  matter  by  volcanic  action,  or  any  other  cause. 
Now,  if  this  region  be  convulsed  by  earthquakes,  the  fissures  f 
g,  and  others  at  right  angles  to  them,  may  sever  the  mass  B 
from  A  and  from  C,  so  that  it  may  move  freely,  and  begin  to 
sink  into  the  chasm.  A  fracture  may  be  conceived  so  clean  and 
perfect  as  to  allow  it  to  subside  at  once  to  the  bottom  of  the  sub- 

*  Conybeare  and  Phillips,  Outlines,  &c.  p.  376. 
t  Phillips,  Geology,  Lardner'a  Cyclop,  p.  41. 


PART  I.     CHAPTER  VI. 


Origin  of  Groat  Faults Denudation  of  Rocks. 

terranean  cavity  ;  but  it  is  far  more  probable  that  the  sinking 
will  be  effected  at  successive  periods  during  different  earthquakes, 
the  mass  always  continuing  to  slide  in  the  same  direction  along 
the  planes  of  the  fissures  f  g,  and  the  edges  of  the  falling  mass 
being  continually  more  broken  and  triturated  at  each  convulsion. 
If,  as  is  not  improbable,  the  circumstances  which  have  caused 
the  failure  of  support  continue  in  operation,  it  may  happen  that 
when  the  mass  B  has  filled  the  cavity  first  formed,  its  foundations 
will  again  give  way  under  it,  so  that  it  will  fall  again  in  the  same 
direction.  But,  if  the  direction  should  change,  the  fact  could  not 
be  discovered  by  observing  the  slickensides,  because  the  last  scor- 
ing would  efface  the  lines  of  previous  friction.  In  the  present 
state  of  our  ignorance  of  the  causes  of  subsidence,  an  hypothesis 
which  can  explain  the  great  amount  of  displacement  in  some 
faults,  on  sound  mechanical  principles,  by  a  succession  of  move- 
ments, is  far  preferable  to  any  theory  which  assumes  each  fault 
to  hwe  been  accomplished  by  a  single  upcast  or  downthrow  of 
several  thousand  feet.  For  we  know  that  there  are  operations 
now  in  progress,  at  great  depths  in  the  interior  of  the  earth,  by 
which  both  large  and  small  tracts  of  ground  are  made  to  rise 
above  and  sink  below  their  former  level,  some  slowly  and  insen- 
sibly, others  suddenly  and  by  starts,  a  few  feet  or  yards  at  a 
time ;  whereas  there  are  no  grounds  for  believing  that,  during 
the  last  3000  years  at  least,  any  regions  have  been  either  up- 
heaved or  depressed,  at  a  single  stroke,  to  the  amount  of  several 
hundred,  much  less  several  thousand  feet. 


CHAPTER  VI. 


DENUDATION,  AND  THE  PRODUCTION  OF  ALLUVIUM. 

Denudation  defined— Its  amount  equal  to  the  entire  mass  of  stratified  deposits 
in  the  earth's  crust — Horizontal  sandstone  denuded  in  Ross-shire — Level  surface 
of  countries  in  which  great  faults  occur — Connexion  of  denudation  and  alluvial 
formations — Alluvium,  how  distinguished  from  rocks  in  situ — Ancient  alluviums 
called  diluvium— Origin  of  these— Erratic  blocks  and  accompanying  gravel- 
Theory  of  their  transportation  by  ice. 

BEFORE  we  take  leave  of  the  aqueous  or  fossiliferous  rocks, 
we  have  still  to  consider  the  alluvial  formations.  Denudation, 
which  has  been  occasionally  spoken  of  in  the  preceding  chapters, 
is  the  removal  of  mineral  matter  by  running  water,  whether  by 


80       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Denudation  of  Stratified  Rocks. 

a  river  or  marine  current,  and  the  consequent  laying  bare  of 
some  inferior  rock.  Geologists  are,  perhaps,  seldom  in  the  habit 
of  reflecting  that  this  operation  is  the  inseparable  accompaniment 
of  the  production  of  all  new  strata  of  mechanical  origin.  The 
transport  of  sediment  and  pebbles,  to  form  a  new  deposit,  neces- 
sarily implies  that  there  has  been,  somewhere  else,  a  grinding 
down  of  rock  into  rounded  fragments,  sand,  or  mud,  equal  in 
quantity  to  the  new  strata.  The  gain  at  one  point  has  merely 
been  sufficient  to  balance  the  loss  at  some  other.  A  ravine,  per- 
haps, has  been  excavated,  or  a  valley  deepened,  or  the  bed  of  the 
sea  has,  by  successive  upheaval,  been  exposed  to  the  power  of 
the  waves,  so  that  part  of  the  superior  covering  of  the  earth's 
crust  has  been  stripped  off,  and  thus  rocks,  previously  hidden, 
have  been  denuded. 

When  we  see  a  stone  building,  we  know  that  somewhere,  far 
or  near,  a  quarry  has  been  opened.  The  courses  of  stone  in  the 
building  may  be  compared  to  successive  strata,  the  quarrjj.to  a 
ravine  or  valley  which  has  suffered  denudation.  As  the  strata, 
like  the  courses  of  hewn  stone,  have  been  laid  one  upon  another 
gradually,  so  the  excavation  both  of  the  valley  and  quarry  have 
been  gradual.  To  pursue  the  comparison  still  farther,  the  super- 
ficial heaps  of  mud,  sand,  and  gravel  usually  called  alluvium, 
may  be  likened  to  the  rubbish  of  a  quarry  which  has  been  re- 
jected as  useless  by  the  workmen,  or  has  fallen  upon  the  road 
between  the  quarry  and  the  building,  so  as  to  lie  scattered  at  ran- 
dom over  the  ground. 

If,  then,  the  entire  mass  of  stratified  deposits  in  the  earth's 
crust  is  at  once  the  monument  and  measure  of  the  denudation 
which  has  taken  place,  on  how  stupendous  a  scale  ought  we  to 
find  the  signs  of  this  removal  of  transported  materials  in  past 
ages  !  Accordingly,  there  are  different  classes  of  phenomena, 
which  attest  in  a  most  striking  manner  the  vast  spaces  left  vacant 
by  the  erosive  power  of  water.  I  may  allude  first,  to  those  val- 
leys on  both  sides  of  which  the  same  strata  are  seen  following 
each  other  in  the  same  order,  and  having  the  same  mineral  com- 
position and  fossil  contents.  We  may  observe  for  example, 
several  formations,  as  No.  1,  2,  3,  4,  in  the  accompanying  dia- 

gram  (Fig  80.);  No.  1.  conglome- 
rate, No.  2.  clay,  No.  3.  grit,  and 
No.  4,  limestone,  each  repeated  in  a 
series  of  hills  separated  by  valleys 
varying  in  depth.  When  we  exa- 
mine the  subordinate  parts  of  these 
four  formations,  we  find,  in  like 


Valleys  of  denudation.  j.   ,.       ,    ,      , 

a.  alluvium.  manner,  distinct  beds  in  each,  cor- 


PART  I.    CHAPTER  VI.  81 

Denudation. 

responding,  on  the  opposite  sides  of  the  valleys,  both  in  compo- 
sition and  order  of  position.  No  one  can  doubt  that  the  strata 
were  originally  continuous,  and  that  some  cause  has  swept  away 
the  portions  which  once  connected  the  whole  series.  A  torrent 
on  the  side  of  a  mountain  produces  similar  interruptions,  and 
when  we  make  artificial  cuts  in  lowering  roads,  we  expose,  in 
like  manner,  corresponding  beds  on  either  side.  But  in  nature, 
these  appearances  occur  in  mountains  several  thousand  feet  high, 
and  separated  by  intervals  of  many  miles  or  leagues  in  extent, 
of  which  a  grand  exemplification  is  described  by  Dr.  MacCul- 
loch,  on  the  north-western  coast  of  Ross-shire,  in  Scotland.* 

Fig:  81. 

SuilVeinn.  CoulUg,    Coutmore. 


Denudation  of  red  sandstone  on  north-west  coast  of  JRoss-skire. 

The  fundamental  rock  of  that  country  is  gneiss,  in  disturbed 
strata,  on  which  beds  of  nearly  horizontal  red  sandstone  rest 
unconformably.  The  latter  are  often  very  thin,  forming  mere 
flags,  with  their  surface  distinctly  ripple-marked.  They  end  ab- 
ruptly on  the  declivities  of  many  insulated  mountains,  which  rise 
up  at  once  to  the  height  of  about  2000  feet  above  the  gneiss  of 
the  surrounding  plain  or  table-land,  and  to  an  average  elevation 
of  about  3000  feet  above  the  sea,  which  all  their  summits  gene- 
rally attain.  The  base  of  gneiss  varies  in  height,  so  that  the 
lower  portions  of  the  sandstone  occupy  different  levels,  and  the 
thickness  of  the  mass  is  various,  sometimes  exceeding  3000  feet. 
It  is  impossible  to  compare  these  scattered  portions  without  ima- 
gining that  the  whole  country  has  once  been  covered  with  a  great 
body  of  sandstone,  and  that  masses  from  1000  to  more  than  3000 
feet  in  thickness  have  been  removed. 

But  perhaps  the  most  convincing  evidence  of  denudation  on  a 
magnificent  scale  is  derived  from  the  levelled  surface  of  many 
districts  in  which  large  faults  occur.  I  have  already  shown,  in 
Fig.  78,  p.  77,  and  in  Fig.  82,  how  angular  and  protruding 
masses  of  rock  might  naturally  have  been  looked  for  on  the  sur- 
face immediately  above  great  faults,  although  in  fact  they  rarely 
exist.  This  phenomenon  may  be  well  studied  in  those  districts 
where  coal  has  been  extensively  worked,  for  there  the  former 
relation  of  the  beds  which  have  shifted  their  position  may  be  de- 
termined with  great  accuracy.  Thus  in  the  coal  field  of  Ashby 
de  la  Zouch,  in  Leicestershire  (see  Fig.  82.),  a  fault  occurs,  on 

*  Western  Islands,  vol.  ii.  p.  89.  pi.  31.  fig.  4. 


82       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Denudation. 


Fig.  82. 


Faults  and  denuded  coal  strata,  Jlshby  de  la  Zouch. 

one  side  of  which  the  coal  beds  abed  rise  to  the  height  of  500 
feet  above  the  corresponding  beds  on  the  other  side.  But  the 
uplifted  strata  do  not  stand  up  500  feet  above  the  general  surface; 
on  the  contrary,  the  outline  of  the  country,  as  expressed  by  the 
line  %  z,  is  uniform  and  unbroken,  and  the  mass  indicated  by  the 
dotted  outline  must  have  been  washed  away.*  There  are  proofs 
of  this  kind  in  some  level  countries,  where  dense  masses  of  strata 
have  been  cleared  away  from  areas  several  hundred  square  miles 
in  extent. 

In  the  Newcastle  coal  district,  it  is  ascertained  that  faults  occur 
in  which  the  upward  or  downward  movement  could  not  have  been 
less  than  140  fathoms,  which,  had  they  affected  equally  the  con- 
figuration of  the  surface  to  that  amount,  would  produce  mountains 
with  precipitous  escarpments  near  1000  feet  high,  or  chasms  of 
the  like  depth  ;  yet  is  the  actual  level  of  the  country  absolutely  uni- 
form, affording  no  trace  whatever  of  subterraneous  disturbance.^ 

The  ground  from  which  these  materials  have  been  removed, 
is  usually  overspread  with  heaps  of  sand  and  gravel,  formed  out  of 
the  ruins  of  the  very  rocks  which  have  disappeared.  Thus,  in  the 
districts  above  alluded  to,  rounded  and  angular  fragments  occur 
of  hard  sandstone,  limestone,  and  ironstone,  with  a  small  quan- 
tity of  the  more  destructible  shale,  and  even  rounded  pieces  of 
coal,  the  form  of  these  relics  pointing  to  water  as.  the  denuding 
agent. 

In  geological  descriptions  we  often  read  of  "alluvium"  and 
"  diluvium,"  as  opposed  to  "  regular  strata,"  or  "  fixed  rocks," 
or  "  rocks  in  situ."  It  will  be  useful,  therefore,  to  explain  these 
terms.  At  the  surface  there  is  commonly  a  layer  of  vegetable 
mould,  derived  partly  from  decayed  plants,  and  partly  caused  by 
the  castings  of  earth-worms,  which  are  continually  sifting  the 

*  See  Mammal's  Geological  Facts,  &c.  p.  90.  and  plate, 
t  Cony beare's Report  to  Brit.  Assoc.  1832.  p.  381. 


PART  I.  CHAPTER  VI. 


Alluvium. 


fine  from  the  coarse  soil.*"  Immediately  beneath  this  mould  the 
regular  or  fundamental  stratified  or  unstratified  rocks  of  the  dis- 
trict may  appear ;  but  there  usually  intervenes,  if  not  an  alluvial 
mass,  at  least  a  quantity  of  broken  and  angular  fragments  of 
the  subjacent  rock,  provincially  called  rubble,  or  brash,  in  many 
parts  of  England.  This  last  may  be  referred  partly  to  the  wea- 
thering or  disintegration  of  stone  on  the  spot,  the  effects  of  air 
and  water,  sun  and  frost,  and  chemical  decomposition,  and  partly 
to  the  expanding  force  of  the  roots  of  trees,  which  may  have 
grown  in  small  crevices,  at  former  geological  periods,  though 
they  may  now  be  wanting.  Sometimes  the  vibratidhs  and  undu- 
lations of  earthquakes  may  have  had  power,  at  some  former  era, 
to  shatter  a  surface  previously  rent  and  weathered.  Thus  in  Ca- 
labria, subterranean  movements  have  been  known  to  throw  up 
into  the  air  the  slabs  of  a  stone  pavement  ;f  and  Mr.  Darwin 
mentions,  that  in  the  Island  of  Quinquina,  in  Chili,  some  narrow 
ridges  of  hard  primary  slate,  which  js  there  the  fundamental  rock, 
were  as  completely  shivered  by  the  vibrations  of  the  great  earth- 
quake of  February,  1835,  as  if  they  had  been  blasted  by  gun- 
powder. The  effect  was  merely  superficial,  and  had  caused 
fresh  fractures  and  displacement  of  the  soil,  the  slate  below  re- 
maining solid  and  uninjured.:}: 

Alluvium  differs  from  the  rubble  or  brash,  just  described,  as 
being  composed  of  sand  and  gravel,  more  or  less  rolled,  in  part 
local,  but  often  in  great  part  formed  of  materials  transported  from 
a  distance.  The  term  is  derived  from  alluvio,  an  inundation, 
or  alluo,  to  wash.  The  gravel  is  rarely  consolidated,  often  un- 
stratified, like  heaps  of  rubbish  shot  from  a  cart,  but  occasion- 
ally divided  into  wavy  and  oblique  layers,  marking  successive 
deposition  from  water.  Such  alluvium  is  strewed  alike  over  in- 
clined and  horizontal  strata,  and  unstratified  rocks  ;  is  most  abun- 
dant in  valleys,  but  also  occurs  in  high  platforms,  and  even  on 
lofty  mountains,  that  of  the  higher  grounds  usually  differing  from 
that  found  at  lower  levels. 

The  inferior  surface  of  an  alluvial  deposit  is  often  very  irregular, 
conforming  to  all  the  inequalities  of  the  subjacent  rock.  (Fig.  83.) 
Occasionally  a  small  mass,  as  at  c,  appears  detached,  and  as  if  in- 
cluded in  the  subjacent  formation.  Such  isolated  portions  are  usu- 
ally sections  of  winding  subterranean  hollows  filled  up  with  allu- 
vium. They  may  have  been  the  courses  of  springs  or  subterra- 

*  See  Proceedings  of  Geol.  Soc.  No.  52.  p.  574.  Darwin  on  Formation  of 
Mould. 

t  See  Principles  of  Geology,  Index,  "  Calabria." 

t  Darwin,  Journal  of  Travels  in  South  America,  &c.,  1832  to  1836,  in  Voyage 
of  H.  M.  S.  Beagle,  p.  370. 


84 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Relation  of  Alluvium  to  regular  Strata. 


Fig.  83. 


nean  streamlets,  which  have  flowed 
through  and  enlarged  natural 
rents ;  or,  when  on  a  small  scale, 
they  may  "be  spaces  which  the 
roots  of  large  trees  have  once  oc- 
cupied, gravel  and  sand  having 
been  introduced  after  their  decay. 
It  is  not  so  easy  as  may  at  first 
appear  to  draw  a  clear  line  of  dis- 
tinction between  the  fixed  rocks,  or 

a.  vegetable  soil.  regular  strata,  (rocks  in  situ^  or  in 

b.  alluvium.  »         \         j     i •       n      •    i 

c.  mass  of  same,  apparently  detach-  pZace,)  and  their  alluvial  covering 

of  travelled  materials.  If  the  bed 

of  a  torrent  or  river  be  dried  up,  we  call  the  gravel,  sand,  and 
mud,  left  in  their  channels,  or  whatever,  during  floods,  they  may 
have  scattered  over  the  neighbouring  plains,  alluvium.  The 
very  same  materials  carried  into  a  lake  or  sea,  where  they  be- 
come sorted  by  water,  and  arranged  in  more  distinct  layers,  are 
termed  regular  strata. 

In  the  same  manner  we  may  contrast  the  gravel,  sand,  and 
broken  shells,  strewed  along  the  path  of  a  marine  current,  with 
strata  formed  by  the  discharge  of  similar  materials,  year  after 
year,  into  a  deeper  and  more  tranquil  part  of  the  sea. 

If  any  fossils  occur,  the  mass  may  still  be  called  alluvial,  pro- 
vided the  fossils  appear  to  have  been  drifted  to  the  spot.  If  any 
of  them,  as,  for.gxample,  freshwater  or  marine  shells,  seem  to 
have  lived  and  died  where  they  are  entombed,  then  the  deposit, 
though  mainly  consisting  of  drift  materials,  should  not  be  termed 
alluvial,  but  a  regular  marine  or  freshwater  formation.  It  is, 
however,  easy  to  perceive  that  passages  must  occur  from  such 
alluvial  to  regular  deposits,  both  in  the  sea  and  the  estuaries  of 
rivers ;  and  it  is  often  most  difficult  to  distinguish  between  them, 
because  organic  remains  have  been  often  obliterated  in  forma- 
tions of  porous  sand,  gravel,  and  loam,  which  allow  rainwater 
to  percolate  freely  through  them. 

After  what  has  been  said  of  the  connexion  of  denudation  and 
alluvium,  the  student  will  expect  to  find  alluviums  of  various 
ages,  and  at  all  heights  above  the  sea,  formed  both  before  and 
during  the  emergence  of  land,  but  always  most  copiously  at 
periods  when  the  level  of  a  country  has  undergone  changes  by 
subterranean  movements  ;  for  then  the  course  of  running  water, 
whether  marine  or  fluviatile,  has  been  most  frequently  deranged, 
and  the  power  of  the  waves  of  the  ocean  has  been  brought  to 
bear  with  the  greatest  effect  against  the  land. 

Before  tfie  doctrine  of  the  rising  and  sinking  of  large  conti- 


PART  I.     CHAPTER  VI.  85 


Ancient  Alluviums  called  Diluvium. 


nental  areas,  whether  insensibly  or  by  a  repetition  of  sudden 
shocks,  was  admitted  as  part  of  the  actual  course  of  nature,  all 
ancient  alluviums  were  classed  by  some  authors  under  the  com- 
mon title  of  "  diluvium,"  and  were  said  not  to  be  due  to  existing 
causes.  To  establish  this  proposition,  it  was  thought  sufficient  to 
demonstrate  that  the  rivers  which  may  now  happen  to  drain  a 
given  district,  could  never,  in  the  course  of  thousands  of  ages, 
have  given  rise  to  the  valleys  of  denudation  in  which  they  now 
flow,  and  that  these  same  rivers  could  never  have  washed  into 
their  present  situations  (often  the  summits  of  hills,  and  high  table- 
lands,) all  the  gravel  and  boulders  evidently  connected  with  for- 
mer denuding  operations.  It  was  therefore  usual  to  refer  the 
"  diluvium"  to  a  deluge,  or  succession  of  deluges,  which  rolled 
with  ti'emendous  violence  over  the  land,  after  it  had  acquired  its 
present  configuration,  and  its  present  height  above  the  sea.  Not 
only  small  gravel,  but  large  blocks  of  stone,  were  supposed  to 
have  been  transported  from  a  distance  by  these  devastating  floods 
or  waves,  and  lodged  upon  the  hill-tops. 

But  rivers,  as  we  have  seen,  are  not  the  only  existing  causes, 
nor  even  the  most  energetic  agents,  by  which  denudation  may  be 
effected.  If  the  upward  movement  of  land  be  very  slow,  the 
waves  may  easily  clear  away  a  stratum  of  yielding  materials  as 
fast  as  they  rise,  and  before  they  reach  the  surface.  Thus,  a 
wide  uninterrupted  expanse  of  denudation  may  take  place,  and 
masses,  many  hundreds  of  feet  or  yards  in  thickness,  may  waste 
away  by  inches  in  the  course  of  thousands  of  centuries.  But  if 
reefs  composed  of  a  more  refractory  stone  should  at  length  rise 
up,  the  breakers,  as  they  foam  over  them,  may  still  tear  off  frag- 
ments, and  roll  them  along  until  the  bottom  of  the  sea  becomes 
strewed  over  with  blocks  and  pebbles.  This  alluvium  of  marine 
origin  will  be  uplifted  when  the  reefs  are  ultimately  converted 
into  land,  and  may  then  constitute  the  covering  of  the  summits 
of  hills,  or  of  elevated  terraces,  or  table-lands.  At  the  same 
time,  this  gravel  may  be  wanting  in  all  valleys  excavated  either 
during  the  rise  of  the  land,  by  currents  of  the  sea  running  be- 
tween islands,  or  eaten  out  or  deepened  by  rivers  after  the  emer- 
gence of  the  land.  At  the  bottom  of  such  more  modern  valleys 
a  distinct  alluvium  will  be  found,  containing,  perhaps,  some  peb- 
bles washed  out  of  the  older  or  upland  gravel,  but  principally 
composed  of  the  ruins  of  rocks  removed  during  the  erosion  of 
the  newer  valleys. 

It  must  be  remembered,  that  when  we  introduce  such  an  hy- 
pothesis, and  take  for  granted  the  rise  of  the  land  out  of  the  sea, 
we  are  merely  supposing  what  we  know,  from  the  discovery  of 
marine  fossils,  to  have  happened  again  and  again,  at  former  periods. 
H 


86       LYELL'S  ELEMENTS  OF  GEOLOGY.  • 


Erratic  Blocks  drifted  by  Ice. 


Erratic  blocks. — The  great  size  of  the  boulders  sometimes 
found  associated  with  ancient  alluviums,  in  places  between  which 
and  the  parent  rock  deep  valleys,  and  even  seas,  now  intervene, 
has  been  thought  by  some  to  offer  insurmountable  objections  to 
any  theory  which  does  not  introduce  causes  of  great  violence  to 
account  for  their  removal.  These  blocks,  called  erratic,  are 
some  of  them  a  few  feet,  others  several  yards,  in  diameter. 
They  are  strewed  by  myriads  over  the  sandy  countries  of  the 
north  of  Germany,  and  parts  of  Sweden,  Denmark,  Finland, 
and  Russia.  Some  of  them  at  least,  must  have  been  carried  into 
their  present  position  since  the  commencement  of  a  very  modern 
geological  period,  for  they  rest,  near  Stockholm,  and  elsewhere, 
on  layers  of  sand  and  marl  containing  shells  of  the  species  now 
inhabiting  the  Baltic. 

Although  these  erratics  are  far  more  numerous  in  northern 
countries,  some  are  met  with  as  far  south  as  the  Swiss  Jura,  hav- 
ing evidently  been  carried  thither  from  the  Alps,  a  chain  which 
is  now  separated  from  the  Jura  by  one  of  the  broadest  and  deepest 
valleys  in  the  world. 

Now  it  is  inconceivable  how  any  velocity  of  water  could  con- 
vey some  of  these  huge  masses,  over  seas  and  valleys,  to  the 
places  where  they  are  now  found;  but  there  is  no  real  difficulty 
in  supposing  them  to  have  been  carried  by  ice,  when  the  lands 
over  which  they  lie  scattered  were  submerged  beneath  the  sea. 

As  the  reader  may  perhaps  be  incredulous  respecting  the  ade- 
quacy of  the  cause  here  alluded  to,  I  shall  enumerate  many  facts 
recently  brought  to  light,  which  incontestably  prove  how  impor- 
tant a  part  ice  plays  in  the  transfer  of  alluvium  from  place  to 
place,  and  especially  of  that  containing  large  masses  of  rock.  I 
must  confine  myself,  however,  to  a  brief  description  of  a  few 
examples,  as  it  is  not  the  object  of  the  present  work  to  treat  at 
large  of  the  changes  illustrative  of  geological  phenomena,  now 
known  to  be  in  progress  on  the  earth. 

First,  in  regard  to  the  distribution  of  erratics ;  they  occur, 
both  in  the  northern  and  southern  hemispheres,  between  the  for- 
tieth parallels  of  latitude  and  the  poles,  but  are  not  met  with  in 
the  intermediate  equatorial  and  warmer  regions.*  This  fact  at 
once  raises  a  presumption  that  the  greater  warmth  of  parts  of 
Asia,  Africa,  and  America,  nearer  the  line,  has  proved  unfavoura- 
ble to  the  transport  of  such  blocks.  On  the  other  hand,  they 
abound  in  the  colder  regions  of  North  America,  from  Canada 
northwards,  as  well  as  in  northern  Europe ;  and  when  we  travel 
southwards,  and  cross  the  Line  in  South  America,  we  fall  in 


k  See  Darwin,  p.  289,  on  some  supposed  exceptions  to  this  general  ruh 


PART  I.     CHAPTER  VI.  87 


Erratic  Blocks  drifted  by  Ice. 


with  them  again  in  Chili  and  Patagonia,  between  lat.  41°  S.  and 
Cape  Horn.*  Here,  then,  we  have  grounds  for  suspecting  that 
a  cold  climate  is  favourable  to  the  production  of  erratics. 

Now  it  is  well  known,  that,  annually,  in  the  Baltic,  stones  are 
moved  by  ice ;  and,  very  recently,  on  the  shores  of  the  Gulf  of 
Finland,  some  large  fragments  were  ascertained  to  have  been 
carried  to  some  distance.  In  spring,  when  the  fringe  of  ice 
which  has  encircled  the  coast  of  the  Gulf  of  Bothnia,  and  many 
parts  of  Sweden,  Norway,  and  Denmark,  during  winter,  breaks 
up,  large  stones,  with  small  gravel  and  ice,  which  have  been 
firmly  frozen  into  a  solid  mass  on  the  beach,  are  floated  off  to  a 
distance.  In  Canada  similar  operations,  but  on  a  grander  scale, 
have  been  noticed  by  Captain  Dayfield.  In  the  river  St.  Law- 
rence, the  loose  ice  accumulates  on  the  shoals  during  winter,  at 
which  season  the  water  is  low.  The  separate  fragments  of  ice 
are  readily  frozen  together  in  a  climate  where  the  temperature  is 
sometimes  30o  below  zero,  and  boulders  become  entangled  with 
them,  so  that  in  the  spring,  when  the  river  rises,  on  the  melting 
of  the  snow,  the  packs  are  floated  off,  frequently  conveying  away 
the  boulders  to  great  distances.  A  single  block  of  granite,  15 
feet  long,  by  10  feet  both  in  width  and  height,  and  which  could 
not  contain  less  than  1500  cubic  feet  of  stone,  was  in  this  way 
moved  down  the  river  several  hundred  yards,  during  the  late 
survey  in  1837.  Heavy  anchors  of  ships,  lying  on  the  shore, 
have  in  like  manner  been  closed  in  and  removed.  In  October, 
1836,  wooden  stakes  were  driven  several  feet  into  the  ground,  at 
one  point  on  the  banks  of  the  St.  Lawrence,  at  high  water  mark, 
and  over  them  were  piled  many  boulders,  as  large  as  the  united 
force  of  six  men  jcould  roll.  The  year  after,  all  the  boulders  had 
disappeared,  and  others  had  arrived,  and  the  stakes  had  been 
drawn  out  and  carried  away  by  the  ice. 

It  has  also  been  observed,  that  ice-islands,  detached  far  to  the 
north,  perhaps  in  Baffin's  Bay,  are  brought  by  the  current,  in 
great  numbers,  down  the  coast  of  Labrador  every  year,  and  are 
often  carried  through  the  straits  of  Belle  Isle,  between  New- 
foundland and  the  continent  of  America,  which,  after  passing 
through  the  straits,  sometimes  float  for  several  hundred  miles  to 
the  southwest,  up  the  Gulf  of  St.  Lawrence,  between  the  40th 
and  50th  degrees  of  N.  latitude.  In  one  of  these  icebergs, 
heaps  of  boulders,  gravel,  and  stones  were  seen. 

A  similar  agency  of  ice  extends  in  the  southern  hemisphere  to 
still  lower  latitudes.  Thus,  for  example,  we  learn  from  Mr. 
Darwin,  that  glaciers  reaching  down  to  the  sea,  occur  at  the 

*  Darwin,  ibid. 


88       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Erratic  Blocks  drifted  by  Ice. 

head  of  all  the  sounds  along  the  western  coast  of  the  southern 
extremity  of  South  America,  in  latitudes  as  low  as  46°,  and  still 
farther,  the  ice  being  covered  with  great  fragments  of  rock.  Al- 
though these  glaciers  come  down  to  the  sea,  the  mountains  from 
which  they  descend  have  only  half  the  altitude  of  the  Alps,  and 
yet  are  equidistant  from  the  equator.  Portions  of  this  South 
American  ice,  charged  with  large  blocks  of  granite,  were  seen  in 
Sir  George  Eyre's  sound,  in  the  same  parallel  of  latitude  as 
Paris,  floating  outwards  to  the  ocean.* 

It  is  therefore  natural  to  suppose  that  masses  of  rock  may  fre- 
quently be  carried  by  icebergs  from  the  foot  of  the  Andes,  in  this 
quarter  of  South  America,  across  deep  channels,  and  stranded 
on  adjacent  islands  in  the  Pacific,  such  as  Chiloe,  on  which  large 
erratics  from  the  Andes  are  actually  seen  ;  and  a  general  eleva- 
tion of  the  mainland,  together  with  the  islands,  accompanied  by 
the  laying  dry  of  the  intervening  sounds,  might  present  to  a 
future  geologist  a  problem  respecting  the  transport  of  blocks,  as 
enigmatical  as  any  which  are  now  encountered  in  Europe.f 

Icebergs  then,  detached  from  glaciers  together  with  coast  ice, 
may  convey,  for  hundreds  of  miles,  pebbles,  boulders,  sand,  and 
mud,  and  let  these  fall  wherever  they  may  chance  to  melt,  on 
submarine  hills  and  valleys.  These,  when  the  land  emerges 
from  the  deep,  may  constitute  some  of  the  far-transported  allu- 
vium which  has  been  ascribed  to  diluvial  agency.^ 

*  Darwin,  p.  283.  t  Ibid,  p.  286. 

I  For  speculations  on  the  causes  of  a  local  and  general  change  of  climate,  de- 
pendent on  fluctuations  in  physical  geography,  and  proofs  of  the  wide  conversion 
of  sea  into  land  in  Europe,  at  periods  comparatively  modern,  see  Princ.  of  Geol. 
booki. 


PART  I.     CHAPTER  VII.  89 


Trap  Rocks,  whence  named. 


CHAPTER  VII. 

VOLCANIC  ROCKS. 

Trap  rocks — Name,  whence  derived — Their  igneous  origin  at  first  doubted — 
Their  general  appearance  and  character — Volcanic  cones  and  craters,  how 
formed — Mineral  composition  and  texture  of  volcanic  rocks — Varieties  of  felspar 
— Hornblende  and  augite — Isomorphism — Rocks,  how  to  be  studied — Basalt, 
greenstone,  trachyte,  porphyry,  scoria,  amygdaloid,  lava,  tuff— Alphabetical  list, 
and  explanation  of  names  and  synonyms,  of  volcanic  rocks — Table  of  the  analy- 
ses of  minerals  most  abundant  in  the  volcanic  and  hypogene  rocks. 

THE  aqueous  or  fossiliferous  rocks  having  now  been  described, 
we  have  next  to  examine  those  which  may  be  called  volcanic,  in 
the  most  extended  sense  of  that  term.  Suppose  a  a  in  the  an- 

Fig.  84. 


a.  Hypogene  formations,  stratified  and  unstratified. 

b.  Aqueous  formations.  c.  Volcanic  rocks. 

nexed  diagram,  to  represent  the  crystalline  formations,  such  as 
the  granitic  and  metamorphic,  b  b  the  fossiliferous  strata,  and 
c  c  the  volcanic  rocks.  These  last  are  sometimes  found,  as  was 
explained  in  the  first  chapter  and  Frontispiece,  breaking  through 
a  and  &,  sometimes  overlying  both,  and  occasionally  alternating 
with  the  strata  b  b.  They  also  are  seen,  in  some  instances,  to 
pass  insensibly  into  the  unstratified  division  of  a,  or  the  Plutonic 
rocks. 

When  geologists  first  began  to  examine  attentively  the  struc- 
ture of  the  northern  and  western  parts  of  Europe,  they  were 
almost  entirely  ignorant  of  the  phenomena  of  existing  volcanos. 
They  found  certain  rocks,  for  the  most  part  without  stratification, 
and  of  a  peculiar  mineral  composition,  to  which  they  gave  differ- 
ent names,  such  as  basalt,  greenstone,  porphyry,  and  amygda- 
loid. All  these,  which  were  recognized  as  belonging  to  one 
family,  were  called  "trap"  by  Bergmann  (from  trappa,  Swe- 
dish, for  a  flight  of  steps) — a  name  since  adopted  very  generally 
into  the  nomenclature  of  the  science ;  for  it  was  observed  that 
many  rocks  of  this  class  occurred  in  great  tabular  masses  of 

H* 


90       LYELL'S  ELEMENTS  OF  GEOLOGY. 

General  Appearance  of  Volcanic  Rocks. 

unequal  extent,  so  as  to  form  a  succession  of  terraces,  or  steps, 
on  the  sides  of  hills.  This  configuration  appears  to  be  derived 
from  two  causes,  first,  the  abrupt  original  terminations  of  sheets 
of  melted  matter,  which  have  spread,  whether  on  the  land  or 
bottom  of  the  sea,  over  a  level  surface.  For  we  know,  in  the 
case  of  lava  flowing  from  a  volcano,  that  a  stream,  when  it  has 
ceased  to  flow,  and  grown  solid,  very  commonly  ends  in  a  steep 
slope,  as  at  o,  Fig.  85.  But,  secondly,  the  step-like  appearance 
arises  more  frequently  from  the  mode 
Fig.  85.  m  which  horizontal  masses  of  igne- 

ous rock,  such  as  b  c,  intercalated 
between  aqueous  strata,  have,  sub- 
sequently to  their  origin,  been  ex- 
posed, at  different  heights,  by  denu- 
dation. Such  an  outline,  it  is  true, 
is  not  peculiar  to  trap  rocks  :  great 

Step-Me  appearance  of  trap.  ^    other 


kinds  of  stone,  often  presenting  similar  terraces  and  precipices  ; 
but  these  are  usually  on  a  smaller  scale  or  less  numerous,  than 
the  volcanic  steps,  or  form  less  decided  features  in  the  landscape, 
as  being  less  distinct  in  structure  and  composition  from  the  asso- 
ciated rocks. 

Although  the  characters  of  trap  rocks  are  greatly  diversified, 
the  beginner  will  easily  learn  to  distinguish  them  as  a  class  from 
the  aqueous  formations.  Sometimes  they  present  themselves,  as 
already  stated,  in  tabular  masses,  which  are  not  divided  into 
strata,  sometimes  in  shapeless  lumps  and  irregular  cones,  form- 
ing small  chains  of  hills.  Often  they  are  seen  in  dikes  or  wall- 
like  masses,  intersecting  fossiliferous  beds.  The  rock  is  occa- 
sionally found  divided  into  columns,  often  decomposing  into  balls 
of  various  sizes,  from  a  few  inches  to  several  feet  in  diameter. 
The  decomposing  surface  very  commonly  assumes  a  coating  of 
a  rusty  iron  colour,  from  the  oxidation  of  ferruginous  matter,  so 
abundant  in  the  traps  in  which  augite  or  hornblende  occur  ;  or, 
in  the  felspathic  varieties  of  trap,  it  acquires  a  white  opaque  coat- 
ing, from  the  bleaching  of  the  mineral  called  felspar.  On  exam- 
ining any  of  these  volcanic  rocks,  where  they  have  not  suffered 
disintegration,  we  rarely  fail  to  detect  a  crystalline  arrangement 
in  one  or  more  of  the  component  minerals.  Sometimes  the  tex- 
ture of  the  mass  is  cellular  or  porous,  or  has  been  porous,  and 
the  cells  have  become  filled  with  carbonate  of  lime,  or  other  in- 
filtrated mineral,  which  has  thus  taken  the  globular  form  of  the 
cells. 

Most  of  the  volcanic  rocks  produce  a  fertile  soil  by  their  dis- 
integration. It  seems  that  their  component  ingredients,  silica, 


PART  I.     CHAPTER  VII.  91 

Cones  and  Craters. 

alumina,  lime,  potash,  iron,  and  the  rest,  are  in  proportions  well 
fitted  for  vegetation.  As  they  do  not  effervesce  with  acids,  a 
deficiency  of  calcareous  matter  might  at  first  have  been  appre- 
hended ;  but  although  carbonate  of  lime  is  rare,  except  in  the 
nodules  of  amygdaloids,  yet  it  will  be  seen  that  lime  sometimes 
enters  largely  in  the  composition  of  augite  and  hornblende.  (See 
Table,  p.  102.) 

In  regions  where  the  eruption  of  volcanic  matter  has  taken 
place  in  the  open  air,  and  where  the  surface  has  never  since  been 
subjected  to  great  aqueous  denudation,  cones  and  craters  are 
strikingly  characteristic.  Many  hundreds  of  these  cones  are 
seen  in  central  France,  in  the  ancient  provinces  of  Auvergne, 
Velay,  and  Vivarais,  where  they  observe,  for  the  most  part,  a 
linear  arrangement,  and  form  chains  of  hills.  Although  none 


Fig.  86. 


Part  of  the  chain  of  extinct  volcanos  called  theMonts  Dome,  Jluvergne. 
(Scrope.) 

of  the  eruptions  have  happened  within  the  historical  era,  the 
streams  of  lava  may  still  be  traced  distinctly  descending  from 
many  of  the  craters,  and  following  the  lowest  levels  of  the  exist- 
ing valleys.  The  origin  of  the  cone  and  crater-shaped  hill  is 
well  understood,  the  growth  of  many  having  been  watched  during 
volcanic  eruptions.  A  chasm  or  fissure  first  opens  in  the  earth, 
from  which  great  volumes  of  steam  and  other  gases  are  evolved. 
The  explosions  are  so  violent  as  to  hurl  up  into  the  air  fragments 
of  broken  stone,  parts  of  which  are  shivered  into  minute  atoms. 
At  the  same  time  melted  stone  or  lava  usually  ascends  through 
the  chimney  or  vent  by  which  the  gases  make  their  escape.  Al- 
though extremely  heavy,  this  lava  is  forced  up  by  the  expansive 
power  of  entangled  gaseous  fluids,  chiefly  steam  or  aqueous  va- 
pour, exactly  in  the  same  manner  as  water  is  made  to  boil  over 
the  edge  of  a  vessel  when  steam  has  been  generated  at  the  bot- 
tom by  heat.  Large  quantities  of  the  lava  are  also  shot  up  into 
the  air,  where  it  separates  into  fragments,  and  acquires  a  spongy 
texture  by  the  sudden  enlargement  of  the  included  gases,  and 
thus  forms  scoria,  other  portions  being  reduced  to  an  impalpable 
powder  or  dust.  The  showering  down  of  the  various  ejected 


92       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Volcanic  Rocks. 


materials  round  the  orifice  of  eruption,  gives  rise  to  a  conical 
mound,  in  which  the  successive  envelopes  of  sand  and  scoriae 
form  layers,  dipping  on  all  sides  from  a  central  axis.  In  the 
mean  time  a  hollow,  called  a  crater,  has  been  kept  open  in  the 
middle  of  the  mound  by  the  continued  passage  upwards  of  steam 
and  other  gaseous  fluids.  The  lava  sometimes  flows  over  the 
edge  of  the  crater,  and  thus  thickens  and  strengthens  the  sides 
of  the  cone ;  but  sometimes  it  breaks  it  down  on  one  side,  and 
often  it  flows  out  from  a  fissure  at  the  base  of  the  hill.  (See 
Fig.  86.) 

I  have  given  a  full  history  and  description  of  the  phenomena 
of  recent  volcanos  in  the  Principles  of  Geology,  and  cannot  re- 
peat them  here,  but  shall  merely  consider  the  characters  of  the 
igneous  rocks  as  they  appear  to  a  geologist  in  the  earth's  crust. 
The  subject  may  be  treated  of  in  the  following  order ;  first,  the 
mineral  composition,  internal  texture,  and  nomenclature  of  vol- 
canic rocks ;  secondly,  the  manner  and  position  in  which  they 
occur  in  the  earth's  crust,  and  their  external  forms ;  arid,  lastly, 
the  connexion  between  the  products  of  modern  volcanos  and  the 
rocks  usually  styled  trappean. 

Mineral  composition  and  texture. — First,  in  regard  to  the 
composition  of  volcanic  rocks,  the  varieties  most  frequently 
spoken  of,  are  basalt,  greenstone,  syenitic  greenstone,  clink- 
stone, claystone,  and  trachyte ;  while  those  founded  chiefly  on 
peculiarities  of  texture,  are  porphyry,  amygdaloid,  lava,  tuff, 
scoria?,  and  pumice.  It  may  be  stated  generally,  that  all  these 
are  mainly  composed  of  two  minerals,  or  families  of  simple 
minerals,  felspar  and  hornblende,  some  almost  entirely  of 
hornblende,  others  of  felspar. 

These  two  minerals  may  be  regarded  as  two  groups,  rather 
than  species.  Felspar,  for  example,  may  be,  first,  common 
felspar,  that  is  to  say,  potash-felspar,  in  which  the  alkali  is  pot- 
ash (see  Table,  p.  102.) ;  or,  secondly,  albite,  that  is  to  say, 
soda-felspar,  where  the  alkali  is  soda  instead  of  potash ;  or, 
thirdly,  Labrador-felspar  (Labradorite),  which  differs  not  only 
in  its  iridescent  hues,  but  also  in  its  angle  of  fracture  or  cleav- 
age, and  its  composition.  We  also  read  much  of  two  other 
kinds,  called  glassy  felspar  and  compact  felspar,  which,  how- 
ever, cannot  rank  as  varieties  of  equal  importance,  for  both  the 
albitic  and  common  felspar  appear  sometimes  in  transparent  or 
glassy  crystals ;  and  as  to  compact  felspar,  it  is  probably  a 
compound  of  a  less  definite  nature,  sometimes  containing,  ac- 
cording to  Dr.  MacCulloch,  both  soda  and  potash. 

The  other  group,  or  hornblende,  consists  principally  of  two 
varieties ;  first,  hornblende,  and,  secondly,  augite,  which  were 


PART  I.     CHAPTER  VII.  93 

Hornblende  and  Augite. 

once  regarded  as  very  distinct,  although  now  some  eminent 
mineralogists  are  in  doubt  whether  they  are  not  one  and  the 
same  mineral,  differing  only  as  one  crystalline  form  of  native 
sulphur  differs  from  another. 

The  history  of  the  changes  of  opinion  on  this  point  is  curious 
and  instructive.  Werner  first  distinguished  augite  from  horn- 
blende ;  and  his  proposal  to  separate  them  obtained  afterwards 
the  sanction  of  Haiiy,  Mohs,  and  other  celebrated  mineralogists. 
It  was  agreed  that  the  form  of  the  crystals  of  the  two  species 
was  different,  and  their  structure,  as  shown  by  cleavage,  that  is 
to  say,  by  breaking  or  cleaving  the  mineral  with  a  chisel,  or  a 
blow  of  the  hammer,  in  the  direction  in  which  it  yields  most 
readily.  It  was  also  found  by  analysis  that  augite  usually  con- 
tained more  lime,  less  alumina,  and  no  fluoric  acid ;  which  last, 
though  not  always  found  in  hornblende,  often  enters  into  its 
composition,  in  minute  quantity.  In  addition  to  these  charac- 
ters, it  was  remarked  as  a  geological  fact,  that  augite  and  horn- 
blende are  very  rarely  associated  together  in  the  same  rock  ;  and 
that  when  this  happened,  as  in  some  lavas  of  modern  date,  the 
hornblende  occurs  in  the  mass  of  the  rock,  where  crystallization 
may  have  taken  place  more  slowly,  while  the  augite  merely  lines 
cavities  where  the  crystals  may  have  been  produced  rapidly.  It 
was  also  remarked,  that  in  the  crystalline  slags  of  furnaces, 
augitic  forms  were  frequent,  the  hornblendic  entirely  absent; 
hence  it  was  conjectured  that  hornblende  might  be  the  result  of 
slow,  and  augite  of  rapid  cooling.  This  view  was  confirmed 
by  the  fact,  that  Mitscherlich  and  Berthier  were  able  to  make 
augite  artificially,  but  could  never  succeed  in  forming  horn- 
blende. Lastly,  Gustavus  Rose  fused  a  mass  of  hornblende  in 
a  porcelain  furnace,  and  found  that  it  did  not,  on  cooling,  assume 
its  previous  shape,  but  invariably  took  that  of  augite.  The  same 
mineralogist  observed  certain  crystals  in  rocks  from  Siberia 
which  presented  a  hornblende  cleavage,  while  they  had  the 
external  form  of  augite. 

If,  from  these  data,  it  is  inferred  that  the  same  substance  may 
assume  the  crystalline  forms  of  hornblende  or  augite  indiffe- 
rently, according  to  the  more  or  less  rapid  cooling  of  the  melted 
mass,  it  is  nevertheless  certain  that  the  variety  commonly  called 
augite,  and  recognized  by  a  peculiar  crystalline  form,  has  usually 
more  lime  in  it,  and  less  alumina,  than  that  called  hornblende, 
although  the  quantities  of  these  elements  do  not  seem  to  be 
always  the  same.  Unquestionably  the  facts  and  experiments 
above  mentioned  show  the  very  near  affinity  of  hornblende  and 
augite ;  but  even  the  convertibility  of  one  into  the  other  by  melt- 
ing and  recrystallizing,  does  not  perhaps  demonstrate  their  abso- 


94       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Theory  of  Isomorphism. 


lute  identity.  For  there  is  often  some  portion  of  the  materials 
in  a  crystal  which  are  not  in  perfect  chemical  combination  with 
the  rest.  Carbonate  of  lime,  for  example,  sometimes  carries 
with  it  a  considerable  quantity  of  silex  into  its  own  form  of  crys- 
tal, the  silex  being  mechanically  mixed  as  sand,  and  yet  not  pre- 
venting the  carbonate  of  lime  from  assuming  the  form  proper  to 
it.  This  is  an  extreme  case,  but  in  many  others  some  one  or 
more  of  the  ingredients  in  a  crystal  may  be  excluded  from  per- 
fect chemical  union ;  and  after  fusion,  when  the  mass  recrys- 
tallizes,  the  same  elements  may  combine  perfectly  or  in  new 
proportions,  and  thus  a  new  mineral  may  be  produced.  Or  some 
one  of  the  gaseous  elements  of  the  atmosphere,  the  oxygen,  for 
example,  may,  when  the  melted  matter  reconsolidates,  combine 
with  some  one  of  the  component  elements. 

The  different  quantity  of  the  impurities  or  refuse  above 
alluded  to,  which  may  occur  in  all  but  the  most  transparent  and 
perfect  crystals,  may  partly  explain  the  discordant  results  at 
which  experienced  chemists  have  arrived  in  their  analysis  of  the 
same  mineral.  For  the  reader  will  find  that  a  mineral  deter- 
mined to  be  the  same  by  its  physical  characters,  crystalline 
form,  and  optical  properties,  has  often  been  declared  by  skilful 
analysers  to  be  composed  of  distinct  elements.  (See  the  Table 
at  p.  102.)  This  disagreement  seemed  at  first  subversive  of  the 
doctrine,  that  there  is  a  fixed  and  constant  relation  between  the 
crystalline  form  and  structure  of  a  mineral,  and  its  chemical 
composition.  The  apparent  anomaly,  however,  which  threat- 
ened to  throw  the  whole  science  of  mineralogy  into  confusion, 
was  in  a  great  degree  reconciled  to  fixed  principles  by  the  dis- 
coveries of  Professor  Mitscherlich  at  Berlin,  who  ascertained 
that  the  composition  of  the  minerals  which  had  appeared  so  va- 
riable, was  governed  by  a  general  law,  to  which  he  gave  the 
name  of  isomorphism  (from  KK>J,  isos,  and  ^op^,  morphe,  form). 
According  to  this  law,  the  ingredients  of  a  given  species  of 
mineral  are  not  absolutely  fixed  as  to  their  kind  and  quality  ; 
but  one  ingredient  may  be  replaced  by  an  equivalent  portion  of 
some  analogous  ingredient.  Thus,  in  augite,  the  lime  may  be  in 
part  replaced  by  portions  of  protoxide  of  iron,  or  of  manganese, 
while  the  form  of  the  crystal,  and  the  angle  of  its  cleavage 
planes,  remain  the  same.  These  vicarious  substitutions,  how- 
ever, of  particular  elements  cannot  exceed  certain  defined  limits. 

Having  been  led  into  this  digression  on  the  recent  progress  of 
mineralogy,  I  may  here  observe  that  the  geological  student  must 
endeavour  as  soon  as  possible  to  familiarize  himself  with  the 
characters  of  five  at  least  of  the  most  abundant  simple  minerals 
of  which  rocks  are  composed.  These  are,  felspar,  quartz,  mica, 


PART  I.     CHAPTER  VII.  95 


How  to  distinguish  Rocks Basalt. 


hornblende,  and  carbonate  of  lime.  This  knowledge  cannot  be 
acquired  from  books,  but  requires  personal  inspection,  and  the 
aid  of  a  teacher.  It  is  well  to  accustom  the  eye  to  know  the 
appearance  of  rocks  under  the  lens.  To  learn  to  distinguish 
felspar  from  quartz  is  the  most  important  step  to  be  first  aimed 
at ;  when  these  occur  in  a  granular  and  uncrystallized  state,  the 
young  geologist  must  not  be '  discouraged  if,  after  considerable 
practice,  he  often  fails  to  distinguish  them  by  the  eye  alone.  If 
the  felspar  is  in  crystals,  it  is  easily  recognized  by  its  cleavage ; 
but  when  in  grains  the  blow-pipe  must  be  used,  for  the  edges  of 
the  grains  can  be  rounded  in  the  flame,  whereas  those  of  quartz 
are  infusible.  If  the  geologist  is  desirous  of  distinguishing  the 
three  varieties  of  felspar  above  enumerated,  or  hornblende  from 
augite,  it  will  often  be  necessary  to  use  the  reflecting  goniometer 
as  a  test  of  the  angle  of  cleavage,  and  shape  of  the  crystal. 
The  use  of  this  instrument  will  not  be  found  difficult. 

The  external  characters  and  composition  of  the  felspars  are 
extremely  different  from  those  of  augite  or  hornblende ;  so  that 
the  volcanic  rocks  in  which  either  of  these  minerals  decidedly 
predominate,  are  easily  recognized.  But  there  are  mixtures  of 
the  two  elements  in  every  possible  proportion,  the  mass  being 
sometimes  exclusively  composed  of  felspar,  at  other  times  solely 
of  augite,  or,  again,  of  both  in  equal  quantities.  Occasionally, 
the  two  extremes,  and  all  the  intermediate  gradations,  may  be 
detected  in  one  continuous  mass.  Nevertheless  there  are  certain 
varieties  or  compounds  which  prevail  so  largely  in  nature,  and 
preserve  so  much  uniformity  of  aspect  and  composition,  that  it 
is  useful  in  geology  to  regard  them  as  distinct  rocks,  and  to  as- 
sign names  to  them,  such  as  basalt,  greenstone,  trachyte,  and 
others,  already  mentioned. 

Basalt. — As  an  example  of  rocks  in  which  augite  greatly 
prevails,  basalt  may  first  be  mentioned.  Although  we  are  more 
familiar  with  this  term  than  with  that  of  any  other  kind  of  trap, 
it  is  difficult  to  define  it,  the  name  having  been  used  so  vaguely. 
It  has  been  very  generally  applied  to  any  trap  rock  of  a  black, 
bluish,  or  leaden-grey  colour,  having  a  uniform  and  compact 
texture.  Most  strictly,  it  consists  of  an  intimate  mixture  of 
augite,  felspar,  and  iron,  to  which  a  mineral  of  an  olive  green 
colour,  called  olivine,  is  often  superadded,  in  distinct  grains  or 
nodular  masses.  The  iron  is  usually  magnetic,  and  is  often  ac- 
companied by  another  metal,  titanium.  Augite  is  the  predo- 
minant mineral,  the  felspar  being  in  much  smaller  proportions. 
There  is  no  doubt  that  many  of  the  fine-grained  and  dark- 
coloured  trap  rocks,  called  basalt,  contain  hornblende  in  the 
place  of  augite ;  but  this  will  be  deemed  of  small  importance 


96       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Greenstone Trachyte Porphyry Amygdaloid. 


after  the  remarks  above  made.  Other  minerals  are  occasionally 
found  in  basalt ;  and  this  rock  may  pass  insensibly  into  almost 
any  variety  of  trap,  especially  into  greenstone,  clinkstone,  and 
wacke,  which  will  be  presently  described. 

Greenstone,  or  Dolerite,  is  usually  defined  as  a  granular  rock, 
the  constituent  parts  of  which  are  hornblende  and  imperfectly 
crystallized  felspar ;  the  felspar  "being  more  abundant  than  in 
basalt,  and  the  grains  or  crystals  of  the  two  minerals  more  dis- 
tinct from  each  other.  This  name  may  also  be  applied  when 
augite  is  substituted  for  hornblende  (the  dolerite  of  some  au- 
thors), or  when  albite  replaces  common  felspar,  forming  the 
rock  sometimes  called  Andesite. 

Syenitic  greenstone. — The  highly  crystalline  compounds  of 
the  same  two  minerals,  felspar  and  hornblende,  having  a  graniti- 
form  texture,  and  with  occasionally  some  quartz  accompanying, 
may  be  called  Syenitic  greenstone,  a  rock  which  frequently 
passes  into  ordinary  trap,  and  as  frequently  into  granite. 

Trachyte. — A  porphyritic  rock  of  a  whitish  or  greyish  colour, 
composed  principally  of  glassy  felspar,  with  crystals  of  the 
same,  generally  with  some  hornblende  and  some  titaniferous 
iron.  In  composition  it  is  extremely  different  from  basalt,  this 
being  a  felspathic,  as  the  other  is  an  augitic,  rock.  It  has  a  pe- 
culiar rough  feel,  whence  the  name  tfpa^uj,  tracKus,  rough.  Some 
varieties  of  trachyte  contain  crystals  of  quartz. 

_.  Porphyry  is   merely  a    certain 

form  of  rock,  very  characteristic 
of  the  volcanic  formations.  When 
distinct  crystals  of  one  or  more 
minerals  are  scattered  through  an 
earthy  or  compact  base,  the  rock  is 
termed  a  porphyry.  (See  Fig.  87.) 
Thus  trachyte  is  porphyritic  ;  for 
in  it,  as  in  many  modern  lavas, 
there  are  crystals  of  felspar ;  but  in 
some  porphyries  the  crystals  are  ol 
augite,  olivine,  or  other  minerals. 
If  the  base  be  greenstone,  basalt,  or 
Porphyry.  pitchstone,  the  rock  may  be  denomi- 

whitt  crystals  of  felspar in  a  dark     nated  greenstone-porphyry,   pitch- 

base  of  hornblende  and  felspar.  &      ,  ,          r       , 

stone-porphyry,  and  so  forth. 

Amygdaloid. — This  is  also  another  form  of  igneous  rock,  ad- 
mitting of  every  variety  of  composition.  It  comprehends  any 
rock  in  which  round  or  almond-shaped  nodules  of  some  mineral, 
such  as  agate,  calcedony,  calcareous  spar,  or  zeolite,  are  scat- 
tered through  a  base  of  wacke,  basalt,  greenstone,  or  other  kind 


PART  1.     CHAPTER  VII. 


97 


Sconce Pumice 


Lava. 


of  trap.  It  derives  its  name  from  the  Greek  word  amygdala, 
an  almond.  The  origin  of  this  structure  cannot  be  doubted,  for 
we  may  trace  the  process  of  its  formation  in  modern  lavas. 
Small  pores  or  cells  are  caused  by  bubbles  of  steam  and  gas 
confined  in  the  melted  matter.  After  or  during  consolidation 
these  empty  spaces  are  gradually  filled  up  by  matter  separating 
from  the  mass,  or  infiltered  by  water  permeating  the  rock.  As 
these  bubbles  have  been  sometimes  lengthened  by  the  flow  of  the 
lava  before  it  finally  cooled,  the  contents  of  such  cavities  have 
the  form  of  almonds.  In  some  of  the  amygdaloidal  traps  of 
Scotland,  where  the  nodules  have  decomposed,  the  empty  cells 
are  seen  to  have  a  glazed  or  vitreous  coating,  and  in  this  respect 
exactly  resemble  scoriaceous  lavas,  or  the  slags  of  furnaces. 

Fig.  88.  The  annexed  figure  re- 

presents a  fragment  of 
stone  taken  from  the  upper 
part  of  a  sheet  of  basaltic 
lava  in  Auvergne.  One 
half  is  scoriaceous,  the 
pores  being  altogether 
empty,  the  other  part  is 
amygdaloidal,  the  pores  or 
cells  being  mostly  filled 
up  with  carbonate  of  lime, 
forming  white  kernels. 

Scoriceand  Pumice  may 
next  be  mentioned  as  po- 
rous rocks,  produced  by 

Swriaceous  lava  in  part  converted  into  an  amyg-    flip  n^finn  nf  rrncpq  rm  ma. 
datad-Montasne  de  la  Veille,  Department  of  me.  *C 

Puy  de  Dome,  France.  tenals  melted  by  volcanic 

heat.  Rcoricz  are  usually  of  a  reddish  brown  and  black  colour, 
and  are  the  cinders  and  slags  of  basaltic  or  augitic  lavas.  Pumice 
is  a  light,  spongy,  fibrous  substance,  produced  by  the  action  of 
gases  on  trachytic  and  other  lavas ;  the  relation,  however,  of  its 
origin  to  the  composition  of  lava  is  not  yet  well  understood.  Von 
Buch  says  it  does  not  occur  where  only  Labrador-felspar  is 
present. 

Lava. — This  term  has  a  somewhat  vague m  signification,  hav- 
ing been  applied  to  all  melted  matter  observed  to  flow  in  streams 
from  volcanic  vents.  When  this  matter  consolidates  in  the  open 
air,  the  upper  part  is  usually  scoriaceous,  and  the  mass  becomes 
more  and  more  stony  as  we  descend,  or  in  proportion  as  it  has 
consolidated  more  slowly  and  under  greater  pressure.  At  the 
bottom,  however,  of  a  stream  of  lava,  a  small  portion  of  scoria- 
ceous rock  very  frequently  occurs,  formed  by  the  first  thin  sheet 
i 


98       LYELL'S  ELEMENTS  OF  GEOLOGY. 

Trap  Tuff Volcanic  Tuff. 

of  liquid  matter,  which  often  precedes  the  main  current,  or  by 
contact  with  water  in  or  upon  the  damp  soil. 

The  more  compact  lavas  are  often  porphyritic,  but  even  the 
scoriaceous  part  sometimes  contains  imperfect  crystals,  which 
have  been  derived  from  some  older  rocks,  in  which  the  crystals 
pre-existed,  but  were  not  melted,  as  being  more  infusible  in  their 
nature. 

Although  melted  matter  rising  in  a  crater,  and  even  that  which 
enters  rents  on  the  side  of  a  crater,  is  called  lava,  yet  this  term 
belongs  more  properly  to  that  which  has  flowed  either  in  the  open 
air  or  on  the  bed  of  a  lake  or  sea.  If  the  same  fluid  has  not 
reached  the  surface,  but  has  been  merely  injected  into  fissures 
below  ground,  it  is  called  trap. 

There  is  every  variety  of  composition  in  lavas ;  some  are 
trachytic,  as  in  the  Peak  of  Teneriffe ;  a  great  number  are  ba- 
saltic, as  in  Vesuvius  and  Auvergne ;  others  are  Andesitic,  as 
those  of  Chili ;  some  of  the  most  modern  in  Vesuvius  consist  of 
green  augite,  and  many  of  those  of  Etna  of  augite  and  Labra- 
dor-felspar.* 

Trap-tuff,  volcanic-tuff. — Small  angular  fragments  of  the 
scorice  and  pumice,  above  mentioned,  and  the  dust  of  the  same, 
produced  by  volcanic  explosions,  form  the  tuffs  which  abound  in 
all  regions  of  active  volcanos,  where  showers  of  these  materials, 
together  with  small  pieces  of  other  rocks  ejected  from  the  crater, 
fall  down  upon  the  land  or  into  the  sea.  Here  they  often  become 
mingled  with  shells,  and  are  stratified.  Such  tuffs  are  some- 
times bound  together  by  a  calcareous  cement,  and  form  a  stone 
susceptible  of  a  beautiful  polish.  But  even  when  little  or  no  lime 
is  present,  there  is  a  great  tendency  in  the  materials  of  ordinary 
tuffs  to  cohere  together. 

Besides  the  peculiarity  of  their  composition,  some  tuffs,  or 
volcanic  grits,  as  they  have  been  termed,  differ  from  ordinary 
sandstones  by  the  angularity  of  their  grains.  When  the  frag- 
ments are  coarse,  the  rock  is  styled  a  volcanic  breccia.  Tufa- 
ceous  conglomerates  result  from  the  intermixture  of  rolled  frag- 
ments or  pebbles  of  volcanic  and  other  rocks  with  tuff. 

According  to  Mr.  Scrope,  the  Italian  geologists  confine  the 
term  tuff,  or  tufa,  to  felspathose  mixtures,  and  those  composed 
principally  of  pumice,  using  the  term  peperino  for  the  basaltic 
tuffs.f 

We  meet  occasionally  with  extremely  compact  beds  of  volca- 
nic materials,  interstratified  with  fossiliferous  rocks,  much  re- 

*G.  Rose,  Ann.  des  Mines,  torn.  8.  p.  32. 
t  Geol.  Trans,  vol.  ii.  p.  211.  Seoend  Series. 


PART  I.     CHAPTER  VII.  99 

Volcanic  Rocks. 

sembling  the  trap  which  may  be  found  in  a  dike.  These  may 
sometimes  be  tuffs,  notwithstanding  their  density  or  compactness. 
The  chocolate-coloured  mud,  which  was  poured  for  weeks  out  of 
the  crater  of  Graham's  Island,  in  the  Mediterranean,  in  1831, 
must,  when  unmixed  with  other  materials,  have  constituted  a 
stone  heavier  than  granite.  Each  cubic  inch  of.  the  impalpable 
powder  which  has  fallen  for  days  through  the  atmosphere  during 
some  modern  eruptions,  has  been  found  to  weigh,  without  being 
compressed,  as  much  as  ordinary  trap  rocks,  which  are  often 
identical  in  mineral  composition. 

The  fusibility  of  the  igneous  rocks  generally  exceeds  that  of 
other  rocks,  for  there  is  much  alkaline  matter  and  lime  in  their 
composition,  which  serves  as  a  flux  to  the  large  quantity  of  silica, 
which  would  be  otherwise  so  refractory  an  ingredient. 

It  is  remarkable,  that  notwithstanding  the  abundance  of  this 
silica,  quartz  is  wanting  in  the  volcanic  rocks,  or  is  present  only 
as  an  occasional  mineral,  like  mica.  The  elements  of  mica,  as 
of  quartz,  occur  in  lava  and  trap,  but  the  circumstances  under 
which  these  rocks  are  formed,  are  evidently  unfavourable  to  the 
development  of  mica  and  quartz,  minerals  so  characteristic  of  the 
hypogene  formations. 

It  would  be  tedious  to  enumerate  all  the  varieties  of  trap  and 
lava  which  have  been  regarded  by  different  observers  as  suffi- 
ciently abundant  to  deserve  distinct  names,  especially  as  each 
investigator  is  too  apt  to  exaggerate  the  importance  of  local  va- 
rieties which  happen  to  prevail  in  districts  best  known  to  him.  It 
will  be  useful,  however,  to  subjoin  here,  in  the  form  of  a  glos- 
sary, an  alphabetical  list  of  the  names  and  synonyms  most  com- 
monly in  use,  with  brief  explanations,  to  which  I  have  added  a 
table  of  the  analysis  of  the  simple  minerals  most  abundant  in 
the  volcanic  and  hypogene  rocks.  f 


Explanation  of  the  names,  synonyms,  and  mineral  composition 
of  the  more  abundant  volcanic  rocks. 

AMPHIBOLITE.  See  Hornblende  rock,amphibole  being  Haiiy's  name  for  horn- 
blende. 

AMYGDALOID.     A  particular  form  of  volcanic  rock;  see  p.  96. 

AUGITE  ROCK.  A  kind  of  basalt  or  greenstone,  composed  wholly  or  principally 
of  granular  augite.  (Leonhord's  Mineralreichs,  2d  edition,  p.  85.) 

AUGITIOPORPHYRY.  Crystals  of  Labrador-felspar  and  of  augite,  in  a  green  or 
dark  grey  base.  (Rose,  Ann.  des  Mines,  torn.  8.  p.  22.  1835.) 

BASALT.  Chiefly  augite — an  intimate  mixture  of  augite  and  felspar  with  mag- 
netic iron,  olivine,  &c.  See  p.  95.  The  yellowish  green  mineral  called 


100      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Mineral  Composition  of  Volcanic  Rocks. 

oiivine,  can  easily  be  distinguished  from  yellowish  felspar  by  its  infusibility 
and  having  no  cleavage.  The  edges  turn  brown  in  the  flame  of  the  blow- 
pipe. 

CL.AYSTO.NE  and  CLAYSTONE-PORPHYRY.  An  earthy  and  compact  stone,  usually 
of  a  purplish  colour,  like  an  indurated  clay;  passes  into  hornstone ;  gene- 
rally contains  scattered  crystals  of  felspar  and  sometimes  of  quartz. 

CLINKSTONE.  Syn.  Phonolite,  fissile  Petrosilex;  a  greenish  or  greyish  rock, 
having  a  tendency  to  divide  into  slabs  and  columns  ;  hard,  with  clean  frac- 
ture, ringing  under  the  hammer ;  principally  composed  of  compact  felspar, 
and,  according  to  Gmelin,  of  felspar  and  mesot5'pe.  (Leonhard,  Mineral- 
reichs, p.  102.)  A  rock  much  resembling  clinkstone,  and  called  by  some 
Petrosilex,  contains  a  considerable  percentage  of  quartz  and  felspar.  As 
both  trachyte  and  basalt  pass  into  clinkstone,  the  rock  so  called  must  be  very 
various  in  composition. 

COMPACT  FELSPAR,  which  has  also  been  called  Petrosilex ;  the  rock  so  called 
includes  the  hornstone  of  some  mineralogists,  is  allied  to  clinkstone,  but  is 
harder,  more  compact,  and  translucent.  It  is  a  varying  rock,  of  which  the 
chemical  composition  is  not  well  defined,  and  is  perhaps  the  same  as  that  of 
clay.  (MacCulloch's  Classification  of  Rocks,  p.  481.)  Mr.  MacCulloch  says, 
that  it  contains  both  potash  and  soda. 

CORNEAN.  A  variety  of  claystone  allied  to  hornstone.  A  fine  homogeneous 
paste,  supposed  to  consist  of  an  aggregate  of  felspar,  quartz,  and  hornblende, 
with  occasionally  epidote,  and  perhaps  chlorite  ;  it  passes  into  compact  fels- 
par and  hornstone.  (De  la  Beche,  Geol.  Trans,  second  series,  vol.  2.  p.  3.) 

DIALLAGE  ROCK.  Sun.  Euphatide,  Gabbro,  and  some  Ophiolites.  "  Compounded 
of  felspar,  and  diallage,  sometimes  with  the  addition  of  serpentine,  or  mica, 
or  quartz.  (MacCulloch,  ibid.  p.  648.) 

DIORITE.  A  kind  of  greenstone,  which  see.  Components,  felspar  and  horn- 
blende in  grains.  According  to  Rose,  Ann.  des  Mines,  tern.  8.  p.  4.,diorile 
consists  of  albite  and  hornblende. 

DIORITIC-PORPHYRY.  A  porphyritic  greenstone,  composed  of  crystals  of  albite 
and  hornblende,  in  a  greenish  or  blackish  base.  (Rose,  ibid.  p.  10.) 

DOLERITE.  Formerly  defined  as  a  synonym  of  greenstone,  which  see.  But 
according  to  Rose  (ibid.  p.  32.),  its  composition  is  black  augite  and  Labrador- 
felspar  ;  according  to  Leonhard  (Mineralreichs,  &c.  p.  77,),  augite,  Labrador- 
felspar,  and  magnetic  iron. 

DOMITE.  An  earthy  condition  of  trachyte,  found  in  the  Puy  de  Dome,  in  Au 
vergne.  , 

EUPHOTIDE.  A  mixture  of  grains  of  Labrador-felspar  and  diallage.  (Rose,  ibid. 
p.  19.)  According  to  some,  this  rock  is  defined  to  be  a  mixture  of  augite  or 
hornblende,  and  Saussurite,  a  mineral  allied  to  jade.  (Allan's  Mineralogy, 
p.  158.)  See  Diallage  rock. 

FELSPAR-PORPHYRY.  Syn.  Hornstone-porphyry ;  a  base  of  felspar,  with  crys- 
tals of  felspar,  and  crystals  and  grains  of  quartz.  See  also  Hornstone. 

GABBRO,  see  Diallage  rock. 

GREENSTONE  ;  Syn.  Dolerite  and  diorite  ;  components,  hornblende  and  felspar, 

or  augite  and  felspar  in  grains.     See  above,  p.  96. 
GREYSTONE.    (Graustein  of  Werner.)    Lead  grey  and  greenish  rock,  composed 

of  felspar  and  augite,  the  felspar  being  more  than  seventy-five  per  cent. 

(Scrope,  Journ.  of  Sci.  No.  42.  p.  221.)    Greystone  lavas  are  intermediate  in 

composition  between  basaltic  and  trachytic  lavas. 

HORNBLENDE  ROCK.  A  greenstone,  composed  wholly  or  principally  of  granular 
hornblende,  or  augite.  (Leonhard,  Mineralreichs,  &c.,  p.  85.) 

HORNSTONE,  HORNSTONE-PORPHYRY.  A  kind  of  felspar-  porphyry,  (Leonhard, 
ibid.)  with  a  base  of  hornstone,  a  mineral  approaching  near  to  flint,  differing 
from  compact  felspar  in  being  infusible. 


PART  I.     CHAPTER  VII.  101 

Mineral  Composition  of  Volcanic  Rocks. 

HYPERSTHENE  ROCK,  a  mixture  of  grains  of  Labrador-felspar  and  hypersthene, 
(Hose,  Ann.  des  Mines,  torn.  8.  p.  13.)  having  the  structure  of  syenite  or 
granite  ;  abundant  among  the  rocks  of  Sky.  In  a  geological  view,  it  has 
been  called  a  greenstone,  in  which  hypersthene  takes  the  place  of  horn- 
blende. 

MELAPHYRE.  A  variety  of  black  porphyry,  the  base  being  black  augite  with 
crystals  of  felspar ;  from  ne\as,  melas,  black. 

OBSIDIAN.     Vitreous  lava  like  melted  glass,  nearly  allied  to  pitchstone. 

OPHIOLITE,  sometimes  same  as  Diallage  rock  (Leonhard,  p.  77.) ;  sometimes  a 
kind  of  serpentine. 

OPHITE.  A  green  porphyritic  rock,  composed  chiefly  of  hornblende,  with  crys- 
tals of  that  mineral  in  a  base  of  the  same,  mixed  with  some  felspar.  It 
passes  into  serpentine  by  a  mixture  of  talc.  (BuraCs  D'Aubuisson,  torn.  2. 
p.  63. 

PEARLSTONE.  A  volcanic  rock  haying  the  lustre  of  mother  of  pearl ;  usually 
having  a  nodular  structure  ;  intimately  related  to  obsidian,  but  less  glassy. 

PEPERINO.     A  form  of  volcanic  tuff!  composed  of  basaltic  scorias.     See  p.  98. 

PETROSILEX.    See  Clinkstone  and  Compact  Felspar. 

PHONOLITE.     Syn.  of  Clinkstone,  which  see. 

PITCHSTONE  ;  vitreous  lava,  less  glassy  than  obsidian ;  a  blackish  green  rock 
resembling  glass,  having  a  resinousjustre  and  appearance  of  pitch  ;  compo- 
sition various,  usually  felspar  and  augite ;  passes  into  basalt ;  occurs  in 
veins,  and  in  Arran  forms  a  dike  thirty  feet  wide,  cutting  through  sand- 
stone ;  forms  the  outer  walls  of  some  basaltic  dikes. 

PORPHYRY.  Any  rock  in  which  detached  crystals  of  felspar,  or  of  one  or  more 
minerals,  are  diffused  through  a  base.  See  p.  96. 

POZZOLANA.     A  kind  of  tuff.     See  p.  53. 

PUMICE.     A  light,  spongy,  fibrous  form  of  trachyte.     See  p.  97. 

PYROXENIC-PORPHYRY,  same  as  augitic-porphyry,  pyroxene  being  Haiiy's  name 
for  augite. 

Syn. 
See  p.  97. 

SERPENTINE.  A  greenish  rock,  in  which  there  is  much  magnesia  ;  usually  con- 
tains diallage,  which  is  nearly  allied  to  the  simple  mineral  called  serpentine. 
Occurs  sometimes,  though  rarely,  in  dikes,  altering  the  contiguous  strata  ;  is 
indifferently  a  member  of  the  trappean  or  hyppgene  series. 

SYENITIC-GREENSTONE  ;  composition,  crystals  or  grains  of  felspar  and  hornblende. 

TEPHRINE,  synonymous  with  lava. 

TOADSTONE.     A  local  name  in  Derbyshire  for  a  kind  of  wacke,  which  see. 

TRACHYTE,  chiefly  composed  of  glassy  felspar,  with  crystals  of  glassy  felspar. 

See  p.  96. 

TRAP  TUFF.     See  p.  98. 
TRASS.    A  kind  of  tuff  or  mud  poured  out  by  lake-craters  during  eruptions : 

common  in  the  Eifel,  in  Germany. 

TUFACEOUS   CONGLOMERATE.      See  p.  98. 

TUFF.     Syn.  Trap-tuff,  volcanic  tuff     See  p.  98. 

VITREOUS  LAVA.    See  Pitchslone  and  Obsidian. 
VOLCANIC  TUFF.    See  p.  98. 

WACKE.  A  soft  and  earthy  variety  of  trap,  having  an  argillaceous  aspect.  It 
resembles  indurated  clay,  and  when  scratched,  exhibits  a  shining  streak. 

WHINSTONE.  A  Scotch  provincial  term  for  greenstone  and  other  hard  trap 
rocks. 


102 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Analysis  of  Minerals.. 


-ss  ^sssx^ 


PART  I.    CHAPTER  VIII. 


103 


Volcanic  Dikes. 


CHAPTER  VIII. 
VOLCANIC  ROCKS  —  continued. 


Trap  dikes — sometimes  project — sometimes  leave  fissures  vacant  by  decom- 
position— Branches  and  veins  of  trap— Dikes  more  crystalline  in  the  centre — 
Foreign  fragments  of  rock  imbedded — Strata  altered  at  or  near  the  contact — Ob- 
literation of  organic  remains — Conversion  of  chalk  into  marble — and  of  coal  into 
coke — Inequality  in  the  modifying  influence  of  dikes — Trap  interposed  between 
strata — Columnar  and  globular  structure — Relation  of  trappean  rocks  to  the  pro- 
ducts of  active  volcanos — Submarine  lava  and  ejected  matter  corresponds  gene- 
rally to  ancient  trap. 

HAVING  in  the  last  chapter  spoken  of  the  composition  and  min- 
eral characters  of  volcanic  rocks,  I  shall  next  describe  the  man- 
ner and  position  in  which  they  occur  in  the  earth's  crust,  and 
their  external  forms.  Now  the  leading  varieties,  such  as  basalt, 
greenstone,  trachyte,  porphyry,  and  th«  rest  are  found  some- 
times in  dikes  penetrating  stratified  and  unstratified  formations, 
sometimes  in  shapeless  masses  protruding  through  or  overlying 
them,  or  in  horizontal  sheets  intercalated  between  strata. 

Volcanic  dikes. — Fissures  have  already  been  spoken  of  as 
occurring  in  all  kinds  of  rocks,  some  a  few  feet,  others  many 
yards  in  width,  and  often  filled  up  with  earth  or  angular  pieces 
of  stone,  or  with  sand  and  pebbles.  Instead  of  such  materials, 
suppose  a  quantity  of  melted  stone  to  be  driven  or  injected  into 
an  open  rent,  and  there  consolidated,  we  have  then  a  tabular 


Fig.  89. 


mass  resembling  a 
wall,  and  called  a 
trap  dike.  It  is  not 
uncommon  to  find 
such  dikes  passing 
through  strata  of  soft 
materials,  such  as  tuff 
or  shale,  which,  being 
more  perishable  than 
the  trap,  are  often 
washed  away  by  the 
sea,  rivers,  or  rain, 
in  which  case  the  dike 

Z»*e  ,n  tnlana  valley,  near  tU  Brmen  Heaa,  Madeira.    gtandg       prominentlv 

out  in  the  face  of  precipices,  or  on  the  level  surface  of  a  country. 
(See  the  annexed  figure.) 


L04 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Trap  Dikes  and  Veins. 


Fig.  90. 


In  the  islands  of  Arran,  Sky,  and  other  parts  of  Scotland, 
where  sandstone,  conglomerate,  and  other  hard  rocks  are  tra- 
versed by  dikes  of  trap,  the  converse  of  the  above  phenomenon 
-is  seen.  The  dike  having  decomposed  more  rapidly  than  the 
containing  rock,  has  once  more  left  open  the  original  fissure, 
often  for  a  distance  of  many  yards  inland  from  the  sea-coast,  as 
represented  in  the  annexed  view.  (Fig.  90.)  In  these  instances 

the  greenstone  of  the  dike  is  usu- 
ally more  tough  and  hard  than  the 
sandstone;  but  chemical  action, 
and  chiefly  the  oxidation  of  the 
iron,  has  given  rise  to  the  more  ra- 
pid decay. 

There  is  yet  another  case,  by  no 
means  uncommon  in  Arran  and 
other  parts  of  Scotland,  where  the 
strata  in  contact  with  the  dike,  and 
for  a  certain  distance  from  it,  have 
been  hardened,  so  as  to  resist  the 
action  of  the  weather  more  than 
the  dike  itself,  or  the  surrounding 
rocks.  When  this  happens,  two 
parallel  walls  of  indurated  strata  are 


Fig.  91. 


Fissures  left  vacant  by  decomposed  trap,  seen  protruding  above  the  general 
Strathaird,  Sky.   (MacCuiioch.)      level  of  the  country,  and  following 

the  course  of  the  dike. 

As  fissures  sometimes  send  off  branches,  or  divide  into  two 
or  more  fissures  of  equal  size,  so  also  we  find  trap  dikes  bifur- 
cating and  ramifying,  and  sometimes  they  are  so  tortuous  as  to 
be  called  veins,  though  this  is  more  common  in  granite  than  in 
trap.  The  accompanying  sketch 
(Fig.  91.)  by  Dr.  MacCuiioch  repre- 
sents part  of  a  sea-cliff  in  Argyleshire, 
where  an  overlying  mass  of  trap,  6, 
sends  out  some  veins  which  terminate 
downwards.  Another  trap  vein,  a  a, 
cuts  through  both  the  limestone,  c, 
and  the  trap,  b. 

In  Fig.  92.  a  ground  plan  is  given 
of  a  ramifying  dike  of  greenstone, 
which  I  observed  cutting  through 
sandstone  on  the  beach  near  Kildonan  Castle,  in  Arran.  The 
larger  branch  varies  from  five  to  seven  feet  in  width,  which  will 
afford  a  scale  of  measurement  for  the  whole. 

In  the  Hebrides  and  other  countries  the  same  masses  of  trap 


Trap  veins  in  Airdnamurchan. 


PART  I.     CHAPTER  VIII. 


105 


Various  forms  of  Trap  Dikes  and  Veins. 


Fig.  92. 


Ground  plan  of  greenstone  dike  traversing  sandstone.    Arran. 

which  occupy  the  surface  of  the  country  far  and  wide,  conceal- 
ing the  subjacent  stratified  rocks,  are  seen  also  in  the  sea-cliffs, 

Fig.  93. 


Trap  dividing  and  covering  sandstone  near  Suishnish  in  Sky.    (MacCulloch.) 


prolonged  downwards  in  veins  or  dikes,  which  probably  unite 
with  other  masses  of  igneous  rock  at  a  greater  depth.  The 
largest  of  the  dikes  represented  in  the  annexed  diagram,  and 
which  are  seen  in  part  of  the  coast  of  Sky,  is  no  less  than  100 
feet  in  width. 

Every  variety  of  trap  rock  is  sometimes  found  in  these  dikes, 
as  basalt,  greenstone,  felspar,  porphyry,  and  more  rarely  tra- 
chyte. The  amygdaloidal  traps  also  occur,  and  even  tuff  and 
breccia,  for  the  materials  of  these  last  may  be  washed  down  into 
open  fissures  at  the  bottom  of  the  sea,  or  during  eruptions  on  the 
land  may  be  showered  into  them  from  the  air. 

Some  dikes  of  trap  may  be  followed  for  leagues  uninterrupt- 
edly in  nearly  a  straight  direction,  as  in  the  north  of  England, 
showing  that  the  fissures  which  they  fill  must  have  been  of  ex- 
traordinary length. 

Dikes  more  crystalline  in  the  centre. — In  many  cases  trap  at 
the  edges  or  sides  of  a  dike  is  less  crystalline  or  more  earthy 
than  in  the  centre,  in  consequence  of  the  melted  matter  having 
cooled  more  rapidly  by  coming  in  contact  with  the  cold  sides  of 
the  fissure ;  whereas,  in  the  centre,  the  matter  of  the  dike  being 
kept  long  in  a  fluid  or  soft  state,  the  crystals  are  slowly  formed. 
In  the  ancient  part  of  Vesuvius  a  thin  band  of  half-vitreous  lava 
is  found  at  the  edge  of  some  dikes.  At  the  junction  of  green- 
stone dikes  with  limestone,  a  sahlband,  or  selvage,  of  serpentine 
is  occasionally  observed. 

On  the  left  shore  of  the  fiord  of  Christiania,  in  Norway,  a  re- 
markable dike  of  syenitic  greenstone  is  traced  through  transition 
strata,  until  at  length,  in  the  promontory  of  Nsesodden,  it  enters 


106 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Foreign  Fragments  in  Dikes. 


Green- 
stone. 


Fig.  94.  mica-schist.    Fig.  94.  repre- 

Sy  enitic  greenstone  dike  of  Nassodden,  Christiania-    sents   a  ground    plan,  where 

the  dike  appears  eight  paces 
in  width.  In  the  middle  it  is 
highly  crystalline  and  gra- 
nitiform,  of  a  purplish  co- 
lour, and  containing  a  few 
crystals  of  mica,  and  strong- 
ly contrasted  with  the  whitish 
mica-schist,  between  which 
and  the  syenitic  rock  there  is 
usually  on  each  side  a  dis- 
tinct black  band,  18  inches  wide,  of  dark  greenstone.  When 
first  seen,  these  bands  have  the  appearance  of  two  accompanying 
dikes  ;  yet  they  are,  in  fact,  only  the  different  form  which  the 
syenitic  materials  have  assumed  where  near  to  or  in  contact  with 
the  mica-schist.  At  one  point,  a,  one  of  the  sahlbands  termi- 
nates for  a  space ;  but  near  this  there  is  a  large  detached  block 
&,  having  a  gneiss-like  structure,  consisting  of  hornblende  and 
felspar,  which  is  included  in  the  midst  of  the  dike.  Round  this 
a  smaller  encircling  zone,  is  seen,  of  dark  basalt,  or  fine-grained 
greenstone,  nearly  corresponding  to  the  larger  ones  which  border 
the  dike,  but  only  one  inch  wide.* 

The  fact  above  alluded  to,  of  a  foreign  fragment,  such  as  b 
(Fig.  94.),  included  in  the  midst  of  the  trap,  as  if  torn  off  from 
some  subjacent  rock  or  the  walls  of  a  fissure,  is  by  no  means 

uncommon.  A  fine  illus- 
tration is  seen  in  a  dike  of 
greenstone,  ten  feet  wide, 
in  the  northern  suburbs  of 
Christiania,  in  Norway,  of 
which  the  annexed  figure 
is  a  ground  plan.  The 
dike  passes  through  shale, 
known  by  its  fossils  to  be- 
long to  the  transition,  or 
Silurian  series.  In  the 
black  base  of  greenstone 
are  angular  and  roundish 
pieces  of  gneiss,  some  white,  others  of  a  light  flesh-colour,  some 
without  lamination,  like  granite,  others  with  laminse,  which,  by 
their  various  and  often  opposite  directions.,  show  that  they  have 


Fi 


Greenstone  dike,  with  fragments  of  gneiss  ; 
Sorgenfri,  Christiania. 


*This  dike  has  been  described  by  Professor  Keilhau,  of  Chrisiiania,  in  whose 
company  I  examined  it. 


PART  I.     CHAPTER  VIII.  107 


Rocks  altered  by  Dikes. 


Deen  scattered  at  random  through  the  matrix.  These  imbedded 
pieces  of  gneiss  measure  from  one  to  about  eight  inches  in 
diameter. 

Rocks  altered  by  volcanic  dikes. — After  these  remarks  on 
the  form  and  composition  of  dikes  themselves,  I  shall  describe 
the  alterations  which  they  sometimes  produce  in  the  rocks  in 
contact  with  them.  The  changes  are  usually  such  as  the  intense 
heat  of  the  melted  matter  and  the  entangled  gases  might  be 
expected  to  cause. 

Plas-Newydd. — A  striking  example,  near  Plas-Newydd,  in 
Anglesea,  has  been  described  by  Professor  Henslow.*  The  dike 
is  1 34  feet  wide,  and  consists  of  a  rock  which  is  a  compound  of 
felspar  and  augite  (dolerite  of  some  authors.)  Strata  of  shale 
and  argillaceous  limestone,  through  which  it  cuts  perpendicu- 
larly, are  altered  to  a  distance  of  thirty,  or  even,  in  some  places, 
to  thirty-five  feet  from  the  edge  of  the  dike.  The  shale,  as  it 
approaches  the  trap,  becomes  gradually  more  compact,  and  is 
most  indurated  where  nearest  the  junction.  Here  it  loses  part 
of  its  schistose  structure,  but  the  separation  into  parallel  layers 
is  still  discernible.  In  several  places  the  shale  is  converted  into 
hard  porcellanous  jasper.  In  the  most  hardened  part  of  the  mass 
the  fossil  shells,  principally  Productce,  are  nearly  obliterated ; 
yet  even  here  their  impressions  may  frequently  be  traced.  The 
argillaceous  limestone  undergoes  analogous  mutations,  losing  its 
earthy  texture  as  it  approaches  the  dike,  and  becoming  granular 
and  crystalline.  But  the  most  extraordinary  phenomenon  is  the 
appearance  in  the  shale  of  numerous  crystals  of  analcime  and 
garnet,  which  are  distinctly  confined  to  those  portions  of  the 
rock  affected  by  the  dike.f  Garnets  have  been  observed,  under 
very  analogous  circumstances,  in  High  Teesdale,  by  Professor 
Sedgwick,  where  they  occur  in  shale  and  limestone,  altered  by 
basalt.f 

Antrim — In  several  parts  of  the  county  of  Antrim,  in  the 
north  of  Ireland,  chalk  with  flints  is  traversed  by  basaltic  dikes. 
The  chalk  is  there  converted  into  granular  marble  near  the 
basalt,  the  change  sometimes  extending  eight  or  ten  feet  from  the 
wall  of  the  dike,  being  greatest  near  the  point  of  contact,  and 
thence  gradually  decreasing  till  it  becomes  evanescent.  "  The 
extreme  effect,"  says  Dr.  Berger,  "  presents  a  dark  brown  crys- 
talline limestone,  the  crystals  running  in  flakes  as  large  as  those 
of  coarse  primitive  (metamorphic)  limestone  ;  the  next  state  is 
saccharine,  then  fine-grained  and  arenaceous ;  a  compact  variety, 

*  Cambridge  Transactions,  vol.  i.  p.  410.  t  Ibid.  vol.  ii.  p.  175. 


108  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Rocks  altered  by  Trap  Dikes. 

having  a  porcellanous  aspect  and  a  bluish-grey  colour,  suc- 
ceeds :  this,  towards  the  outer  edge,  becomes  yellowish-white, 
and  insensibly  graduates  into  the  unaltered  chalk.  The  flints  in 
the  altered  chalk  usually  assume  a  grey  yellowish  colour."  *  All 
traces  of  organic  remains  are  effaced  in  that  part  of  the  lime- 
stone which  is  most  crystalline. 

The   annexed  drawing  (Fig.  96.)  represents  three  basaltic 
dikes  traversing  the  chalk,  all  within  the  distance  of  ninety  feet. 

Fig.  96. 


Clialk 


Dike  35  ft.        Dike  Dike  20  ft. 

1  foot. 

Basaltic  dikes  in  chalk  in  island  of  Rathlin,  Antrim. — Ground  plan  as  seen  on  the 
bead).    (Conybeare  and  Buckland.)f 

The  chalk  contiguous  to  the  two  outer  dikes  is  converted  into  a 
finely  granular  marble,  m  m,  as  are  the  whole  of  the  masses 
between  the  outer  dikes  and  the  central  one.  The  entire  con- 
trast in  the  composition  and  colour  of  the  intrusive  and  invaded 
rocks,  in  these  cases,  renders  the  phenomena  peculiarly  clear 
and  interesting. 

Another  of  the  dikes  of  the  north-east  of  Ireland  has  con- 
verted a  mass  of  red  sandstone  into  hornstone4  By  another, 
the  slate  elay  of  the  coal  measures  has  been  indurated,  and  has 
assumed  the  character  of  flinty  slate ;  §  and  in  another  place  the 
slate  clay  of  the  lias  has  been  changed  into  flinty  slate,  which 
still  retains  numerous  impressions  of  ammonites.|| 

It  might  have  been  anticipated  that  beds  of  coal  would,  from 
their  combustible  nature,  be  affected  in  an  extraordinary  degree 
•by  the  contact  of  melted  rock.  Accordingly,  one  of  the  green- 
stone dikes  of  Antrim,  on  passing  through  a  bed  of  coal,  re- 
duces it  to  a  cinder  for  the  space  of  nine  feet  on  each  side.lT 

At  Cockfield  Fell,  in  the  north  of  England,  a  similar  change 
is  observed.  Specimens  taken  at  the  distance  of  about  thirty 
yards  from  the  trap  are  not  distinguishable  from  ordinary  pit 
coal ;  those  nearer  the  dike  are  like  cinders,  and  have  all  the 
*  . . . 

*  Dr.  Berger,  Geol.  Trans.,  First  Series,  vol.  iii.  p.  172. 

t  Geol.  Trans.,  First  Series,  vol.  iii.  p.  210.  and  plate  10. 

t  Ibid.  p.  201.  $  Ibid.  p.  205. 

II  Ibid.  p.  213. ;  and  Playfair,  Illust  of  Hutt  Theory,  s.  253.     T  Ibid.  p.  206. 


PART  I.     CHAPTER  VIII.  109 


Rocks  altered  by  Trap  Dikes. 


character  of  coke ;  while  those  close  to  it  are  converted  into  a 
substance  resembling  soot.* 

As  examples  might  be  multiplied  without  end,  I  shall  merely 
select  one  or  two  others,  and  then  conclude.  The  rock  of  Stir- 
ling Castle  is  a  calcareous  sandstone,  fractured,  and  forcibly  dis- 
placed by  a  mass  of  greenstone,  which  has  evidently  invaded 
the  strata  in  a  melted  state.  The  sandstone  has  been  indurated, 
and  has  assumed  a  texture  approaching  to  hornstone  near  the 
junction.  In  Arthur's  Seat  and  Salisbury  Craig,  near  Edin- 
burgh, a  sandstone  which  comes  in  contact  with  greenstone,  is 
converted  into  a  jaspideous  rock.f 

The  secondary  sandstones  in  Sky  are  converted  into  solid 
quartz  in  several  places,  where  they  come  in  contact  with  veins 
or  masses  of  trap ;  and  a  bed  of  quartz,  says  Dr.  MacCulloch, 
found  near  a  mass  of  trap,  among  the  coal  strata  of  Fife,  was 
in  all  probability  a  stratum  of  ordinary  sandstone,  having  been 
subsequently  indurated  and  turned  into  quartzite  by  the  action 
of  heat4 

But  although  strata  in  the  neighbourhood  of  dikes  are  thus 
altered  in  a  variety  of  cases,  shale  being  turned  into  flinty  slate 
or  jasper,  limestone  into  crystalline  marble,  sandstone  into 
quartz,  coal  into  coke,  and  the  fossil  remains  of  all  such  strata 
wholly  or  in  part  obliterated,  it  is  by  no  means  uncommon  to 
meet  with  the  same  rocks,  even  in  the  same  districts,  absolutely 
unchanged  in  the  proximity  of  volcanic  dikes. 

This  great  inequality  in  the  effects  of  the  igneous  rocks  may 
often  arise  from  an  original  difference  in  their  temperature,  and 
in  that  of  the  entangled  gases,  such  as  is  ascertained  to  prevail 
in  different  lavas,  or  in  the  same  lava  near  its  source  and  at  a 
distance  from  it.  The  power  also  of  the  invaded  rocks  to  con- 
duct heat  may  vary,  according  to  their  composition,  structure, 
and  the  fractures  which  they  may  have  experienced,  and  per- 
haps, also,  according  to  the  quantity  of  water  (so  capable  of 
being  heated)  which  they  contain.  It  must  happen  in  some 
cases  that  the  component  materials  are  mixed  in  such  propor- 
tions as  prepare  them  readily  to  enter  into  chemical  union,  and 
form  new  minerals ;  while  in  other  cases  the  mass  may  be  more 
homogeneous,  or  the  proportions  less  adapted  for  such  union. 

We  must  also  take  into  consideration,  that  one  fissure  may  be 
simply  filled  with  lava,  which  may  begin  to  cool  from  the  first ; 

*  Sedgwick,  Camb.  Trans.,  vol.  ii.  p.  37. 

t  Illust.  of  Hutt.  Theory,  §  253.  and  261.  Dr.  MacCulloch,  Geol.  Trans.,  First 
Series,  vol.  ii.  p.  305. 

J  Syst.  of  Geol.,  vol.  i.  p.  206. 
K 


110      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Forcibly  intruded  Trap. 

whereas  in  other  cases  the  fissure  may  give  passage  to  a  current 
of  melted  matter,  which  may  ascend  for  days  or  months,  feed- 
ing streams  which  are  overflowing  the  country  above,  or  are 
ejected  in  the  shape  of  scoriae  from  some  crater.  If  the  walls 
of  a  rent,  moreover,  are  heated  by  hot  vapour  before  the  lava 
rises,  as  we  know  may  happen  on  the  flanks  of  a  volcano,  the 
additional  caloric  supplied  by  the  dike  and  its  gases  will  act 
more  powerfully. 

Intrusion  of  trap  between  strata.— In  proof  of  the  mechanical 
force  which  the  fluid  trap  has  sometimes  exerted  on  the  rocks 
into  which  it  has  intruded  itself,  I  may  'refer  to  the  Whin-Sill, 
where  a  mass  of  basalt,  from  sixty  to  eighty  feet  in  height,  re- 
presented by  a,  Fig.  97.,  is  in  part  wedged  in  between  the  rocks 
of  limestone,  b,  and  shale,  c,  which  have  been  separated  from 
the  great  mass  of  limestone  and  shale,  d,  with  which  they  were 
united. 


Fig.  97. 


Trap  interposed  between  displaced  beds  of  limestone  and  shale,  at  White,  Force,  High 
Teesdale,  Durham.    (Sedgwick.)* 

The  shale  in  this  place  is  indurated ;  and  the  limestone,  which 
at  a  distance  from  the  trap  is  blue,  and  contains  fossil  corals,  is 
here  converted  into  granular  marble  without  fossils. 

Masses  of  trap  are  not  unfrequently  met  with  intercalated  be- 
tween strata,  and  maintaining  their  parallelism  to  the  planes  of 
stratification  throughout  large  areas.  They  must  in  some  places 
have  forced  their  way  laterally  between  the  divisions  of  the  strata, 
a  direction  in  which  there  would  be  the  least  resistance  to  an  ad- 
vancing fluid,  if  no  vertical  rents  communicated  with  the  surface, 
and  a  powerful  hydrostatic  pressure  was  caused  by  gases  pro- 
pelling the  lava  upwards. 

Columnar  and  globular  structure. — One  of  the  characteristic 
forms  of  volcanic  rocks,  especially  of  basalt,  is  the  columnar, 
where  large  masses  are  divided  into  regular  prisms,  sometimes 
separable,  but  in  other  cases  adhering  firmly  together.  The 
columns  vary  in  the  number  of  angles,  from  three  to  twelve ; 

*Gamb.  Trans.,  vol.  ii.  p.  180. 


PART  I.    CHAPTER  VIII. 


Ill 


Columnar  Structure  of  Volcanic  Rocks. 


but  they  have  most  commonly  from  five  to  seven  sides.  They 
are  often  divided  transversely,  at  nearly  equal  distances,  like  the 
joints  in  a  vertebral  column,  as  in  the  Giant's  Causeway,  in  Ire- 
land. They  vary  exceedingly  in  respect  to  length  and  diameter. 
Dr.  MacCulloch  mentions  some  in  Sky  which  are  about  400  feet 
long ;  others,  in  Morven,  not  exceeding  an  inch.  In  regard  to 
diameter,  those  of  Ailsa  measure  nine  feet,  and  those  of  Morven 
an  inch  or  less.*  They  are  usually  straight,  but  sometimes 
curved  ;  and  examples  of  both  these  occur  in  the  island  of  StafFa. 
In  a  horizontal  bed  or  sheet  of  trap  the  columns  are  vertical ;  in 
a  vertical  dike  they  are  horizontal.  Among  other  examples  of 
the  last-mentioned  phenomenon  is  the  mass  of  basalt,  called  the 
Chimney,  in  St.  Helena  (See  Fig. 
98.),  a  pile  of  hexagonal  prisms,  64 
feet  high,  evidently  the  remainder 
of  a  narrow  dike,  the  walls  of  rock 
which  the  dike  originally  traversed 
having  been  removed  down  to  the 
level  of  the  sea.  In  Fig.  99.  a  small 
portion  of  this  dike  is  represented  on 
a  less  reduced  scale,  f 

It  being  assumed  that  columnar 
trap  has  consolidated  from  a  fluid 
state,  the  prisms  are  said  to  be  al- 
ways at  right  angles  to  the  cooling 
surfaces.  If  these  surfaces,  there- 


Fig.  98. 


Volcanic  dike  composed  of  horizontal  fore,  instead  of  being  either  perpen- 
prisms.   St.  Helena.  dicular  or  horizontal,  are  curved,  tho 


Fig.  99. 


columns  ought  to  be  inclined  at  every  angle 
to  the  horizon  ;  and  there  is  a  beautiful  ex- 
emplification of  this  phenomenon  in  one  of 
the  valleys  of  the  Vivarais,  a  mountainous 
district  in  the  South  of  France,  where,  in  the 
midst  of  a  region  of  gneiss,  a  geologist  en- 
counters unexpectedly  several  volcanic  cones 
of  loose  sand  and  scorise.  From  the  crater 
of  one  of  these  cones,  called  La  Coupe 
d'Ayzac,  a  stream  of  lava  descends  and  occupies  the  bottom  of 
a  narrow  valley,  except  at  those  points  where  the  river  Volant, 
or  the  torrents  which  join  it,  have  cut  away  portions  of  the  solid 
lava.  The  accompanying  sketch  (Fig.  100.)  represents  the  rem- 
nant of  the  lava  at  one  of  the  points  where  a  lateral  torrent  joins 


Small  portion  of  the  dike 
Fig.  98. 


*  MacCulloch,  Syst.  of  Geol.  vol.  ii.  p.  137. 
t  Scale's  Geognosy  of  St.  Helena,  plate  9. 


112 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Columnar  Structure  of  Volcanic  Rocks. 


Fig.  100. 


Lava  of  La  Coupe  d'rfyzac,  near  dntraigue,  in  the  Province  of  Jirdechc. 

the  main  valley  of  the  Volant.  It  is  clear  that  the  lava  once 
filled  the  whole  valley  up  to  the  dotted  line  da,'  but  the  river 
has  gradually  swept  away  all  below  that  line,  while  the  tributary 
torrent  has  laid  open  a  transverse  section ;  by  which  we  per- 
ceive, in  the  first  place,  that  the  lava  is  composed,  as  usual  in 
this  country,  of  three  parts ;  the  uppermost  at  a,  being  scoriace- 
ous ;  the  second,  6,  presenting  irregular  prisms ;  and  the  third, 
c,  with  regular  columns,  which  are  vertical  on  the  banks  of  the 
Volant,  where  they  rest  on  a  horizontal  base  of  gneiss,  but  which 
are  inclined  at  an  angle  of  45°  at  g,  and  then  horizontal  at  f, 
their  position  having  been  every  where  determined,  according  to 
the  law  before  mentioned,  by  the  concave  form  of  the  original 
valley. 

In  the  annexed  figure  (101.)  a 
view  is  given  of  some  of  the  in- 
clined and  curved  columns  which 
present  themselves  on  the  sides  of 
the  valleys  in  the  hilly  region  north 
of  Vicenza,  in  Italy,  and  at  the  foot 
of  the  higher  Alps.*  Unlike  those 
of  the  Vivarais,  last  mentioned,  the 
basalt  of  this  country  was  evidently 
submarine,  and  the  present  val- 
leys have  since  been  hollowed  out 
by  denudation. 

The  columnar  structure  is  by  no 
means  peculiar  to  the  trap  rocks  in 
which  hornblende  or  augite  predo- 
minate ;  it  is  also  observed  in  clink- 
stone, trachyte,  and  other  felspathic  rocks  of  the  igneous  class, 


Fig.  101. 


Columnar  basalt  in  the  Vicentin. 
(Fortis.) 


*  Fortia,  Mem.  sur  1'Hist  Nat.  de  1'Italie,  torn.  i.  p.  233.  plate  7. 


PART  I.     CHAPTER  VIII. 


113 


Columnar  and  Globular  Structure. 


although  in  these  it  is  rarely  exhibited  in  such  regular  polygonal 
forms. 

It  has  been  already  stated  that  basaltic  columns  are  often  di- 
vided by  cross  joints.  Sometimes  each  segment,  instead  of  an 
angular,  assumes  a  spheroidal  form,  so  that  a  pillar  is  made  up 
of  a  pile  of  balls,  usually  flattened,  as  in  the  Cheese-grotto  at  Ber- 
trich-Baden,  in  the  Eifel,  near  the  Moselle.  (Fig.  102.)  The 

Fig.  102. 


Basaltic  pillars  of  the  Kiisegrotte,  Bertrich- Baden,  halfway  between  Trevet  and 
Coblentz.    Height  of  grotto  from  7  to  8  feet. 

basalt,  there,  is  part  of  a  small  stream  of  lava,  from  30  to  40 
feet  thick,  which  has  proceeded  from  one  of  several  volcanic  cra- 
ters, still  extant,  on  the  neighbouring  heights.  The  position  of 
the  lava  bordering  the  river  in  this  valley,  might  be  represented 
by  a  section  like  that  already  given  (Fig.  100.  p.  112.),  if  we 
merely  suppose  inclined  strata  of  slate  and  the  argillaceous  sand- 
stone called  greywacke  to  be  substituted  for  gneiss. 

In  some  masses  of  decomposing  greenstone,  basalt,  and  other 
trap  rocks,  the  globular  structure  is  so  conspicuous  that  the  rock 
has  the  appearance  of  a  heap  of  large  cannon-balls. 

A  striking  example  of  this  structure  occurs  in  a  resinous  tra- 
chyte or  pitchstone-porphyry  in  one  of  the  Ponza  islands,  which 
rise  from  the  Mediterranean,  off  the  coast  of  Terracina  and 
Gaieta.  The  globes  vary  from  a  few  inches  to  three  feet  in  di- 
ameter, and  are  of  an  ellipsoidal  form.  (See  Fig.  103.)  The 
whole  rock  is  in  a  state  of  decomposition,  "  and  when  the  balls," 
says  Mr.  Scrope,  "  have  been  exposed  a  short  time  to  the  wea- 
ther, they  scale  off  at  a  touch  into  numerous  concentric  coats, 
like  those  of  a  bulbous  root,  inclosing  a  compact  nucleus.  The 


114 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Relation  of  Trap,  Lava,  and  Scoriae. 


Fig.  103. 


Globiform pitchstone.  Chiaja  di  Luna, 
Isle  of  Ponza.    (Scrope.) 


laminee  of  this  nucleus  have  not  been 
so  much  loosened  by  decomposition ; 
but  the  application  of  a  ruder  blow  will 
produce  a  still  further  exfoliation.* 
A  fissile  texture  is  occasionally 
assumed  by  clinkstone  and  other 
trap  rocks,  so  that  they  have  been 
used  for  roofing  houses.  Some- 
times the  prismatic  and  slaty  struc- 
ture is  found  in  the  same  mass. 
The  causes  which  give  rise  to  such 
arrangements  are  very  obscure,  but 
are  supposed  to  be  connected  with 
changes  of  temperature  during  the 
cooling  of  the  mass,  as  will  be 
pointed  out  in  the  sequel.  (See 
Chap.  X.) 

Relation  of  trappean  rocks  to  the 
products  of  active  volcanos. 

I  When  we  reflect  on  the  changes 
above  described  in  the  strata  near  their  contact  with  trap  dikes, 
and  consider  how  great  is  the  analogy  in  composition  and  struc- 
ture of  the  rocks  called  trappean  and  the  lavas  of  active  volca- 
nos, it  seems  difficult  at  first  to  understand  how  so  much  doubt 
could  have  prevailed  for  half  a  century  as  to  whether  trap  was 
of  igneous  or  aqueous  origin.  To  a  certain  extent,  however, 
there  was  a  real  distinction  between  the  trappean  formations  and 
those  to  which  the  term  volcanic  was  almost  exclusively  con- 
fined. The  trappean  rocks  first  studied  in  the  north  of  Germany, 
and  in  Norway,  France,  Scotland,  and  other  countries,  were 
either  such  as  had  been  formed  entirely  under  deep  water,  or  had 
been  injected  into  fissures  and  intruded  between  strata,  and  which 
had  never  flowed  out  in  the  air,  or  over  the  bottom  of  a  shallow 
sea.  When  these  products,  therefore,  of  submarine  or  subterra- 
nean igneous  action  were  contrasted  with  loose  cones  of  scoriae, 
tuff,  and  lava,  or  with  narrow  streams  of  lava  in  great  part  sco- 
riaceous  and  porous,  such  as  were  observed  to  have  proceeded 
from  Vesuvius  and  Etna,  the  resemblance  seemed  remote  and 
equivocal.  It  was,  in  truth,  like  comparing  the  roots  of  a  tree 
with  its  leaves  and  branches,  which,  although  they  belong  to  the 
same  plant,  differ  in  form,  texture,  colour,  mode  of  growth,  and 
position.  The  external  cone,  with  its  loose  ashes  and  porous 


*  Scrope,  Gcol.  Trans,  vol.  ii.  p.  205.  Second  Series. 


PART  I.    CHAPTER  VIII.  115 


Relation  of  Trap,  Lava,  and  Scorise. 


lava,  may  be  likened  to  the  light  foliage  and  branches,  and  the 
rocks  concealed  far  below,  to  the  roots.  But  it  is  not  enough  to 
say  of  the  volcano, 

"  quantum  vertice  in  auras 
"  ^Etherias,  tantum  radice  in  Tartara  tendit," 

for  its  roots  do  literally  reach  downwards  to  Tartarus,  or  to  the 
regions  of  subterranean  fire ;  and  what  is  concealed  far  below, 
is  probably  always  more  important  in  volume  and  extent  than 
what  is  visible  above  ground. 

We  have  already  stated  how  frequently  dense  masses  of  strata 
have  been  removed  by  denudation  from  wide  areas-  (see  Chap. 
VI.)  ;  and  this  fact  prepares  us  to  expect  a  similar  destruction  of 
whatever  may  once  have  formed  the  uppermost  part  of  ancient 
submarine  or  subaerial  volcanos,  more  especially  as  those  super- 
ficial parts  are  always  of  the  lightest  and  most  perishable  mate- 
rials. The  abrupt  manner  in  which  dikes  of  trap  usually  termi- 

nate  at  the  surface  (see  Fig.  104.), 
and  the  water-worn  pebbles  of  trap 
m  the  alluvium  which  covers  the 
dike,  prove  incontestably  that  what- 
ever was  uppermost  in  these  forma- 
tions has  been  swept  away.  It  is 
easy,  therefore  to  conceive  that 
what  is  gone  in  regions  of  trap  may 
have  corresponded  to  what  is  now 

Strata  intersected  by  a  trap  dike,  and    visible  in  active  VOlcattOS.       . 

covered  with  alluvium.  It  will  be  shown  in  the  second  part 

of  this  volume,  that  in  the  earth's  crust  there  are  volcanic  tuffs 
of  all  ages,  containing  marine  shells,  which  bear  witness  to  erup- 
tions at  many  successive  geological  periods.  These  tuffs,  and  the 
associated  trappean  rocks,  must  not  be  compared  to  lava  and  sco- 
ria3  which  had  cooled  in  the  open  air.  Their  counterparts  must 
be  sought  in  the  products  of  modern  submarine  volcanic  eruptions. 
If  it  be  objected  that  we  have  no  opportunity  of  studying  these  last, 
it  may  be  answered,  that  subterranean  movements  have  caused, 
almost  everywhere  in  regions  of  active  volcanos,  great  changes  in 
the  relative  level  of  land  and  sea,  in  times  comparatively  modern, 
so  as  to  expose  to  view  the  effects  of  volcanic  operations  at  the 
bottom  of  the  sea. 

Thus,  for  example,  the  recent  examination  of  the  igneous 
rocks  of  Sicily,  especially  those  of  the  Val  di  Noto,  has  proved 
that  all  the  more  ordinary  varieties  of  European  trap  have  been 
there  produced  under  the  waters  of  the  sea,  at  a  modern  period ; 


116  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Relation  of  Trap,  Lava,  and  Scoriae. 

that  is  to  say,  since  the  Mediterranean  has  been  inhabited  by  a 
great  proportion  of  the  existing  species  of  testacea. 

These  igneous  rocks  of  the  Val  di  Noto,  and  the  more  ancient 
trappean  rocks  of  Scotland  and  other  countries,  differ  from  sub- 
aerial  volcanic  formations  in  being  more  compact  and  heavy, 
and  in  forming  sometimes  extensive  sheets  of  matter  intercalated 
between  marine  strata,  and  sometimes  stratified  conglomerates, 
of  which  the  rounded  pebbles  are  all  trap.  They  differ  also  in 
the  absence  of  regular  cones  and  craters,  and  in  the  want  of  con- 
formity of  the  lava  to  the  lowest  levels  of  existing  valleys. 

It  is  highly  probable,  however,  that  insular  cones  did  exist  in 
some  parts  of  the  Val  di  Noto ;  and  that  they  were  removed  by 
the  waves,  in  the  same  manner  as  the  cone  of  Graham  Island, 
in  the  Mediterranean,  was  swept  away  in  1831,  and  that  of 
Nyoe,  off  Iceland,  in  1783.  All  that  would  remain  in  such 
cases,  after  the  bed  of  the  sea  has  been  upheaved  and  laid 
dry,  would  be  dikes  and  shapeless  masses  of  igneous  rock,  cut- 
ting through  sheets  of  lava,  which  may  have  spread  over  the 
level  bottom  of  the  sea,  and  strata  of  tuff,  formed  of  materials 
first  scattered  far  and  wide  by  the  winds  and  waves,  and  then 
deposited.  Trap  conglomerates  also,  to  which  the  action  of  the 
waves  must  give  rise  during  the  denudation  of  such  volcanic 
islands,  will  emerge  from  the  deep  whenever  the  bottom  of  the 
sea  becomes  land.* 

The  proportion  of  volcanic  matter  which  is  originally  subma- 
rine must  always  be  very  great,  as  those  volcanic  vents  which 
are  not  entirely  beneath  the  sea,  are  almost  all  of  them  in 
islands,  or,  if  on  continents,  near  the  shore.  This  may  explain 
why  extended  sheets  of  trap  so  often  occur,  instead  of  narrow 
threads,  like  lava  streams.  For,  a  multitude  of  causes  tend,  near 
the  land,  to  reduce  the  bottom  of  the  sea  to  a  nearly  uniform 
level, — the  sediment  of  rivers, — materials  transported  by  the 
waves  and  currents  of  the  sea  from  wasting  cliffs, — showers  of 
sand  and  scoriae  ejected  by  volcanos,  and  scattered  by  the  wind 
and  waves.  When,  therefore,  lava  is  poured  out  on  such  a  sur- 
face, it  will  spread  far  and  wide  in  every  direction  in  a  liquid 
sheet,  which  may  afterwards,  when  raised  up,  form  the  tabular 
capping  of  the  land. 

As  to  the  absence  of  porosity  in  the  trappean  formations,  the 
appearances  are  in  a  great  degree  deceptive,  for  all  amygdaloids 


*  See  Principlesof  Geology,  Index,  "  Graham  Island,"  "  Nyb'e,"  "  Conglome- 
rates, volcanic."  &c. 


PART  I.    CHAPTER  VIII.  117 

Volcanic  Rocks. 

are,  as  already  explained,  porous  rocks,  into  the  cells  of  which 
mineral  matter,  such  as  silex,  carbonate  of  lime,  and  other  in- 
gredients, have  been  subsequently  introduced.  (See  p.  97.) 

In  the  little  Cumbray,  one  of  the  Western  Islands,  near  Arran, 
the  amygdaloid  sometimes  contains  elongated  cavities  filled  with 
brown  spar ;  and  when  the  nodules  have  been  washed  out,  the 
interior  of  the  cavities  is  glazed  with  the  vitreous  varnish  so  cha- 
racteristic of  the  pores  of  slaggy  lavas.  Even  in  some  parts  of 
this  rock  which  are  excluded  from  air  and  water,  the  cells  are 
empty,  and  seem  to  have  always  remained  in  this  state,  and  are 
therefore  undistinguishable  from  some  modern  lavas.* 

Dr.  MacCulloch,  after  examining  with  great  attention  these 
and  the  other  igneous  rocks  of  Scotland,  observes,  "  that  it  is  a 
mere  dispute  about  terms,  to  refuse  to  the  ancient  eruptions  of 
trap  the  name  of  submarine  volcanos  ;  for  they  are  such  in  every 
essential  point,  although  they  no  longer  eject  fire  and  smoke."")" 
The  same  author  also  considers  it  not  improbable  that  some  of 
the  volcanic  rocks  of  the  same  country  may  have  been  poured 
out  in  the  open  air.:}: 

Although  the  principal  component  minerals  of  subaerial  lavas 
are  the  same  as  those  of  intrusive  trap,  and  both  the  columnar 
and  globular  structure  are  common  to  both,  there  are,  neverthe- 
less, some  volcanic  rocks  which  never  occur  as  lava,  such  as 
greenstone,  clinkstone,  the  more  crystalline  porphyries,  and  all 
those  traps  in  which  quartz  and  mica  frequently  appear  as  constitu- 
ent parts.  In  short  the  intrusive  trap  rocks,  forming  the  intermedi- 
ate step  between  lava  and  the  plutonic  rocks,  depart  in  their  char- 
acters from  lava  in  proportion  as  they  approximate  to  granite. 

These  views  respecting  the  relations  of  the  volcanic  and  trap 
rocks  will  be  better  understood,  when  the  reader  has  studied,  in 
the  next  chapter,  what  is  said  of  the  plutonic  formations. 

*  MacCulloch,  West.  Isl.,  vol.  ii.  p.  487. 

t  Syst.  of  Geol.,  vol.  ii.  p.  114.  t  Ibid. 


118      LYELL'S  ELEMENTS  OF  GEOLOGY. 


Plutonic  Rocks. 


CHAPTER  IX. 


PLUTONIC   ROCKS  —  GRANITE. 

General  aspect  of  granite — Decomposing  into  spherical  masses — Rude  colum- 
nar structure  —  Analogy  and  difference  of  volcanic  and  plutonic  formations — 
Minerals  in  granite,  and  their  arrangement — Graphic  and  porphyritic  granite — 
Occasional  minerals — Syenite — Syenitic,  talcose,  and  schorly  granites — Eurite — 
Passage  of  granite  into  trap — Examples  near  Christiania  and  in  Aberdeenshire — 
Analogy  in  composition  of  trachyte  and  granite  —  Granite  veins  in  Glen  Tilt, 
Cornwall,  the  Valorsine,  and  other  countries  —  Different  composition  of  veins 
from  main  body  of  granite — Metalliferous  veins  in  strata  near  their  junction  with 
granite — Apparent  isolation  of  nodules  of  granite — Quartz  veins — Whether  plu- 
tonic rocks  are  ever  overlying — Their  exposure  at  the  surface  due  to  denuda- 
tion. 

THE  plutonic  rocks  may  be  treated  of  next  in  order,  as  they 
are  most  nearly  allied  to  the  volcanic  class,  already  considered. 
I  have  described,  in  the  first  chapter,  these  plutonic  rocks  as  the 
unstratified  division  of  the  crystalline  or  hypogerie  formations, 
and  have  endeavoured  to  point  out  in  the  Frontispiece,  at  D,  the 
position  which  they  occupy,  when  first  formed,  relatively  to  the 
volcanic  formations,  B. 

By  some  writers  all  the  rocks  now  under  consideration  have 
been  comprehended  under  the  name  of  granite,  which  is,  then, 
understood  to  embrace  a  large  family  of  crystalline  and  corn- 
Fig.  105. 


Mass  of  granite  near  the  Sharp  Tor,  Cornwall. 

pound  rocks,  usually  found  underlying  all  other  formations; 
whereas  we  have  seen  that  trap  very  commonly  overlies  strata 
of  different  ages.  Granite  often  preserves  a  very  uniform  char- 
acter throughout  a  wide  range  of  territory,  forming  hills  of  a  pe- 


PART  I.     CHAPTER  IX. 


119 


General  Aspect  of  Granite. 


culiar  rounded  form,  usually  clad  with  a  scanty  vegetation.  The 
surface  of  the  rock  is  for  the  most  part  in  a  crumbling  state,  and 
the  hills  are  often  surmounted  by  piles  of  stones  like  the  remains 
of  a  stratified  mass,  as  in  the  annexed  figure,  and  sometimes  like 
heaps  of  boulders,  for  which  they  have  been  mistaken.  The  ex- 
terior of  these  stones,  originally  quadrangular,  acquires  a  round- 
ed form  by  the  action  of  air  and  water,  for  the  edges  and  angles 
waste  away  more  rapidly  than  the  sides.  A  similar  spherical 
structure  has  already  been  described  as  characteristic  of  basalt, 
and  other  volcanic  formations,  and  it  must  be  referred  to  analo- 
gous causes,  as  yet  but  imperfectly  understood. 

Although  it  is  the  general  peculiarity  of  granite  to  assume  no 
definite  shapes,  it  is  nevertheless  occasionally  subdivided  by  fis- 
sures, so  as  to  assume  a  cuboidal,  and  even  a  columnar,  struc- 
ture. Examples  of  these  appearances  may  be  seen  near  the 
Land's  End,  in  Cornwall,  (see  figure.) 

Fig.  106. 


Granite  having  a  cuboidal  and  rude  columnar  structure. 
Land's  End,  Cornwall. 

The  plutonic  formations  also  agree  with  the  volcanic,  in  hav- 
ing veins  or  ramifications  proceeding  from  central  masses  into 
the  adjoining  rocks,  and  causing  alterations  in  these  last,  which 
will  be  presently  described.  They  also  resemble  trap  in  con- 
taining no  organic  remains ;  but  they  differ  in  being  more  uni- 


120 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Mineral  Composition  of  Granitic  Rocks. 


form  in  texture,  whole  mountain  masses  of  indefinite  extent  ap- 
pearing to  have  originated  under  conditions  precisely  similar. 
But  they  differ  in  never  being  scoriaceous  or  amygdaloidal,  in 
never  forming  a  porphyry  with  an  uncrystalline  base,  and  never 
alternating  with  tuffs.  Nor  do  they  form  conglomerates,  al- 
though there  is  sometimes  an  insensible  passage  from  a  fine  to  a 
coarse  grained  granite,  and  occasionally  patches  of  a  fine  tex- 
ture are  imbedded  in  a  coarser  variety. 

Felspar,  quartz,  and  mica  are  usually  considered  as  the  mine- 
rals essential  to  granite,  the  felspar  being  most  abundant  in  quan- 
tity, and  the  proportion  of  quartz  exceeding  that  of  mica.  These 
minerals  are  united  in  what  is  termed  a  confused  crystallization ; 
that  is  to  say,  there  is  no  regular  arrangement  of  the  crystals  in 
granite  as  in  gneiss  (see  Fig.  107.),  except  in  the  variety  termed 

Fig.  107. 


Gneiss.    (See  description,  ji.  132.) 

graphic  granite,  which  occurs  mostly  in  granitic  veins.  This 
variety  is  a  compound  of  felspar  and  quartz,  so  arranged  as  to 
produce  an  imperfect  laminar  structure.  The  crystals  of  felspar 


Fig.  108. 


Fig.  109. 


OrapJiic  granite. 

•     Fig.  108.     Section  parallel  to  the  laminae. 
Fig.  109.     Section  transverse  to  the  laming. 

appear  to  have  been  first  formed,  leaving  between  them  the  space 
now  occupied  by  the  darker  coloured  quartz.  This  mineral, 
when  a  section  is  made  at  right  angles  to  the  alternate  plates  of 


PART  I.     CHAPTER  IX.  121 

Porphyritic  Granite Syenite. 

felspar  and  quartz,  presents  broken  lines,  which  have  been  com- 
pared to  Hebrew  characters. 

Porphyritic  granite. — This  name  has  been  sometimes  given 
to  that  variety  in  which  large  crystals  of  felspar,  sometimes  more 
than  an  inch  in  length,  are  scattered  through  an  ordinary  base 
of  granite.  An  example  of  this  texture  may  be  seen  in  the  gra- 
nite of  the  Land's  End,  in  Cornwall.  (Fig.  110.)  The  two 

Fig.  110. 


Porphyritic  granite,  Land's  End,  Cornwall. 

larger  prismatic  crystals  in  this  drawing  represent  felspar, 
smaller  crystals  of  which  are  also  seen,  similar  in  form,  scattered 
through  the  base.  In  this  base  also  appear  black  specks  of  mica, 
the  crystals  of  which  have  a  more  or  less  perfect  hexagonal  out- 
line. The  remainder  of  the  mass  is  quartz,  the  translucency  of 
which  is  strongly  contrasted  to  the  opaqueness  of  the  white  fel- 
spar and  black  mica.  But  neither  this  transparency  of  the 
quartz,  nor  the  silvery  lustre  of  the  mica,  can  be  expressed  in 
the  engraving. 

The  uniform  mineral  character  of  large  masses  of  granite 
seems  to  indicate  that  large  quantities  of  the  component  elements 
were  thoroughly  mixed  up  together,  and  then  crystallized  under 
precisely  similar  conditions.  There  are,  however,  many  acci- 
dental, or  "  occasional,"  minerals,  as  they  are  termed,  which  be- 
long to  granite.  Among  these  black  schorl  or  tourmaline,  acti- 
nolite,  zircon,  garnet,  and  fluor  spar,  are  not  uncommon ;  but 
they  are  too  sparingly  dispersed  to  modify  the  general  aspect  of 
the  rock.  They  show,  nevertheless,  that  the  ingredients  were 
not  every  where  exactly  the  same ;  and  a  still  greater  variation 
may  be  traced  in  the  ever-varying  proportions  of  the  felspar, 
quartz,  and  mica. 

Syenite. — When  hornblende  is  the  substitute  for  mica,  which 
is  very  commonly  the  case,  the  rock  becomes  Syenite :  so  called 
from  the  celebrated  ancient  quarries  of  Syene  in  Egypt.  It  has 
all  the  appearance  of  ordinary  granite,  except  when  mineralogi- 


122  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Syenitic  Granite  ....  Takose  Granite  ....  Schorl  Rock  ....  Eurite  ....  Pegmatite. 

cally  examined  in  hand  specimens,  and  being  fully  entitled  to 
rank  as  a  geological  member  of  the  same  plutonic  family  as 
granite.  Syenite,  however,  after  maintaining  the  granitic  cha- 
racter throughout  extensive  regions,  is  not  uncommonly  found 
to  lose  its  quartz,  and  to  pass  insensibly  into  syenitic-greenstone, 
a  rock  of  the  trap  family. 

Syenitic-granite. — The  quadruple  compound  of  quartz,  fel- 
spar, mica,  and  hornblende,  may  be  so  termed.  This  rock  occurs 
in  Scotland  and  in  Guernsey. 

Talcose  granite,  or  Protogine  of  the  French,  is  a  mixture  of 
felspar,  quartz,  and  talc.  It  abounds  in  the  Alps,  and  in  some 
parts  of  Cornwall,  producing  by  its  decomposition  the  china  clay, 
more  than  12,000  tons  of  which  are  annually  exported  from  that 
county  for  the  potteries.* 

Schorl  rock,  and  schorly  granite. — The  former  of  these  is 
an  aggregate  of  schorl,  or  tourmaline,  and  quartz.  When  fel- 
spar and  mica  are  also  present,  it  may  be  called  schorly  granite. 
This  kind  of  granite  is  comparatively  rare. 

Eurite. — A  rock  in  which  all  the  ingredients  of  granite  are 
blended  into  a  finely  granular  mass.  Crystals  of  quartz  and 
mica  are  sometimes  scattered  through  the  base  of  Eurite. 

Pegmatite. — A  name  given  by  the  French  writers  to  a  variety 
of  granite  ;  a  granular  mixture  of  quartz  and  felspar ;  frequent 
in  granite  veins  ;  passes  into  graphic  granite. 

All  these  granites  pass  into  certain  kinds  of  trap,  a  circum- 
stance which  affords  one  of  many  arguments  in  favour  of  what 
is  now  the  prevailing  opinion,  that  the  granites  are  also  of  igneous 
origin.  The  contrast  of  the  most  crystalline  form  of  granite,  to 
that  of  the  most  common  and  earthy  trap,  is  undoubtedly  great ; 
but  each  member  of  the  volcanic  class  is  capable  of  becoming 
porphyritic,  and  the  base  of  the  porphyry  may  be  more  and 
more  crystalline,  until  the  mass  passes  to  the  kind  of  granite 
most  nearly  allied  in  mineral  composition. 

The  minerals  which  constitute  alike  the  granitic  and  volcanic 
rocks,  consist,  almost  exclusively,  of  seven  elements,  namely, 
silica,  alumina,  magnesia,  lime,  soda,  potash,  and  iron  ;  and  these 
may  sometimes  exist  in  about  the  same  proportions  in  a  porous 
lava,  a  compact  trap,  or  a  crystalline  granite.  It  may  perhaps 
be  found,  on  farther  examination,  for  on  this  subject  we  have  yet 
much  to  learn,  that  the  presence  of  these  elements  in  certain  pro- 
portions is  more  favourable  than  in  others  to  their  assuming  a 
crystalline  or  true  granitic  structure;  but  it  is  also  ascertained 
by  experiment,  that  the  same  materials  may,  under  different  cir- 

*  Boase  on  Primary  Geology,  p.  16. 


PART  I.     CHAPTER  IX.  123 


Passage  of  Granite  into  Trap Granite  Veins. 


cumstances,  form  very  different  rocks.  The  same  lava,  for 
example,  may  be  glassy,  or  scoriaceous,  or  stony,  or  porphyritic, 
according  to  the  more  or  less  rapid  rate  at  which  it  cools ;  and 
some  trachytes  and  syenitic-greenstones  may  doubtless  form  gra- 
nite and  syenite,  if  the  crystallization  take  place  slowly. 

It  would  be  easy  to  multiply  examples  and  authorities  to  prove 
the  gradation  of  the  granitic  into  the  trap  rocks.  On  the  western 
side  of  the  fiord  of  Christiania,  in  Norway,  there  is  a  large  dis- 
trict of  trap,  chiefly  greenstone-porphyry,  and  syenitic-green- 
stone,  resting  on  fossiliferous  strata.  To  this,  on  its  southern 
limit,  succeeds  a  region  equally  extensive  of  syenite,  the  passage 
from  the  volcanic  to  the  plutonic  rock  being  so  gradual  that  it  is 
impossible  to  draw  a  line  of  demarcation  between  them. 

"  The  ordinary  granite  of  Aberdeenshire,"  says  Dr.  MacCul- 
loch,  "  is  the  usual  ternary  compound  of  quartz,  felspar,  and 
mica  ;  but  sometimes  hornblende  is  substituted  for  the  mica.  But 
in  many  places  a  variety  occurs  which  is  composed  simply  of 
felspar  and  hornblende ;  and  in  examining  more  minutely  this 
duplicate  compound,  it  is  observed  in  some  places  to  assume  a 
fine  grain,  and  at  length  to  become  undistinguishable  -from  the 
greenstones  of  the  trap  family.  It  also  passes  in  the  same  unin- 
terrupted manner  into  a  basalt,  and  at  length  into  a  soft  clay- 
stone,  with  a  schistose  tendency  on  exposure,  in  no  respect 
differing  from  those  of  the  trap  islands  of  the  western  coast."* 
The  same  author  mentions,  that  in  Shetland,  a  granite  composed 
of  hornblende,  mica,  felspar,  and  quartz,  graduates  in  an  equally 
perfect  manner  into  basalt,  f 

In  Hungary  there  are  varieties  of  trachyte,  which,  geologi- 
cally speaking,  are  of  modern  origin,  in  which  crystals,  not  only 
of  mica,  but  of  quartz,  are  common,  together  with  felspar  and 
hornblende.  It  is  easy  to  conceive  how  such  volcanic  masses 
may,  at  a  certain  depth  from  the  surface,  pass  downwards  into 
granite. 

I  have  already  hinted  at  the  close  analogy  in  the  forms  of  cer- 
tain granitic  and  trappean  veins  ;  and  it  will  be  found  that  strata 
penetrated  by  plutonic  rocks  have  suffered  changes  very  similar 
to  those  exhibited  near  the  contact  of  volcanic  dikes.  Thus,  in 
Glen  Tilt,  in  Scotland,  alternating  strata  of  limestone  and  argil- 
laceous schist  come  in  contact  with  a  .mass  of  granite.  The 
contact  does  not  take  place  as  might  have  been  looked  for,  if  the 
granite  had  been  formed  there  before  the  strata  were  deposited, 
in  which  case,  the  section  would  have  appeared  as  in  Fig.  111.; 
but  the  union  is  as  represented  in  Fig.  112.,  the  undulating  .out- 
line of  the  granite  intersecting  different  strata,  and  occasionally 

*  Sy«t.  of  Geol.,  vol.  i.  p.  157.  t  Ibid.,  p.  158. 


124 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Rocks  altered  by  Granite  Veins. 


Fig.  111. 


Junction  of  granite  and  argillaceous  schist  in  Glen 
Tilt.    (MacCulloch.)* 

intruding  itself  in  tortuous  veins  into  the  beds  of  clay-slate  and 
limestone,  from  which  it  differs  so  remarkably  in  composition. 
The  limestone  is  sometimes  changed  in  character  by  the  prox- 


Fig.  113. 


Junction  of  granite  and  limestone  in  Glen  Tilt. 
a.  Granite.  b.  Limestone. 

c.  Blue  argillaceous  schist. 

imity  of  the  granitic  mass  or  its  veins,  and  acquires  a  more  com- 
pact texture,  like  that  of  hornstone  or  chert,  with  a  splintery  frac- 
ture, effervescing  feebly  with  acids. 

*  Geol.  Trans.,  First  Series,  vol.  iii.  pi.  21. 


PART  I.     CHAPTER  IX. 


125 


Rocks  altered  by  Granite  Veins. 


The  annexed  diagram  (Fig.  113.)  represents  another  junction, 
in  the  same  district,  where  the  granite  sends  forth  so  many  veins 
as  to  reticulate  the  limestone  and  schist,  the  veins  diminishing 
towards  their  termination  to  the  thickness  of  a  leaf  of  paper  or  a 
thread.  In  some  places  fragments  of  granite  appear  entangled, 
as  it  were,  in  the  limestone,  and  are  not  visibly  connected  with 
any  larger  mass ;  while  sometimes,  on  the  other  hand,  a  lump 
of  the  limestone  is  found  in  the  midst  of  the  granite.  The  ordi- 
nary colour  of  the  limestone  of  Glen  Tilt  is  lead  blue,  and  its 
texture  large-grained  and  highly  crystalline ;  but  where  it  ap- 
proximates to  the  -granite,  particularly  where  it  is  penetrated  by 
the  smaller  veins,  the  crystalline  texture  disappears,  and  it  as- 
sumes an  appearance  exactly  resembling  that  of  hornstone.  The 
associated  argillaceous  schist  often  passes  into  hornblende-slate, 
where  it  approaches  very  near  to  the  granite.* 

The  conversion  of  the  limestone  in  these  and  many  other 
instances  into  a  siliceous  rock,  effervescing  slowly  with  acids, 
would  be  difficult  of  explanation,  were  it  not  ascertained  that  such 
limestones  are  always  impure,  containing  grains  of  quartz,  mica, 
or  felspar  disseminated  through  them.  The  elements  of  these 
minerals,  when  the  rock  has  been  subjected  to  great  heat,  may 
have  been  fused,  and  so  spread  more  uniformly  through  the 
whole  mass. 

In  the  plutonic,  as  in  the  volcanic  rocks,  there  is  every  grada- 
tion from  a  tortuous  vein  to  the  most  regular  form  of  a  dike,  such 
as  intersect  the  tuffs  and  lavas  of  Ve- 
suvius and  Etna.  Dikes  of  granite  may 
be  seen,  among  other  places,  on  the 
southern  flank  of  Mount  Battock,  one 
of  the  Grampians,  the  opposite  walls 
sometimes  preserving  an  exact  paral- 
lelism for  a  considerable  distance. 

As  a  general  rule,  however,  granite 
veins  in  all  quarters  of  the  globe  are 
more  sinuous  in  their  course  than  those 
of  trap.  They  present  similar  shapes 
at  the  most  northern  point  of  Scotland, 
and  the  southernmost  extremity  of  Af- 
^  rica,  as  the  annexed  drawing  will  show. 

Granite   veins    traversing    clay         It    is    not    uncommon    for    One  set  of 

Cape°f  granite  veins  to  intersect  another;  and 
sometimes  -there  are  three  sets,  as  in 


*  MacCulloch,  Geol.  Trans.,  vol.  iii.  p.  259. 

t  Capt.  B.  Hall,  Trans.  Roy.  Soc.  Edin.,  vol.  vii. 


126 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Granite  Veins. 


the  environs  of  Heidelberg,  where  the  granite  on  the  hanks  of 
the  river  Necker  is  seen  to  consist  of  three  varieties,  differing  in 
colour,  grain,  and  various  peculiarities  of  mineral  composition. 
One  of  these,  which  is  evidently  the  second  in  age,  is  seen  to  cut 
through  an  older  granite ;  and  another,  still  newer,  traverses 
both  the  second  and  the  first. 

In  Shetland  there  are  two  kinds  of  granite.  One  of  them, 
composed  of  hornblende,  mica,  felspar,  and  quartz,  is  of  a  dark 
colour,  and  is  seen  underlying  gneiss.  The  other  is  a  red  gra- 
nite, which  penetrates  the  dark  variety  every  where  in  veins.* 

The  accompanying  sketches  will  explain  the  manner  in  which 
granite  veins  often  ramify  and  cut  each  other.  (Figs.  115.  and 

116.)  They  represent  the 
manner  in  which  the  gneiss 
at  Cape  Wrath,  in  Suther- 
landshire,  is  intersected  by 
veins.  Their  light  colour, 
strongly  contrasted  with  that 
of  the  hornblende-schist, 
here  associated  with  the 
gneiss,  renders  them  very 


Fig.  115. 


conspicuous. 

Granite  very  generally 
assumes  a  finer  grain,  and 
undergoes  a  change  in  min- 
eral composition,  in  the  veins 
which  it  sends  into  contigu- 
ous rocks.  Thus,  according  to  Professor  Sedgwick,  the  main 
body  of  the  Cornish  granite  is  an  aggregate  of  mica,  quartz,  and 

Fig.  116. 


Granite  veins  traversing  gneiss,  Cape  Wrath. 
(MacCultoch.)t 


Granite  veins  traversing  gneiss  at  Cape  Wrath,  in  Scotland.    (MacCulloch.) 

felspar ;  but  the  veins  are  sometimes  without  mica,  being  a  gra- 


*  MacCulloch,  Syst.  of  Geol.,  vol.  i.  p.  58. 
t  Western  Islands,  pi.  31. 


PART  I.    CHAPTER  IX. 


127 


Mineral  Structure  of  Granite  in  Veins. 


nular  aggregate  of  quartz  and  felspar.  In  other  varieties  quartz 
prevails  to  the  almost  entire  exclusion  both  of  felspar  and  mica ; 
in  others,  the  mica  and  quartz  both  disappear,  and  the  vein  is 
simply  composed  of  white  granular  felspar.* 


Fig.  117. 


Granite  veins  passing  through  hornblende  slate,  Carnsilver  Cove,  Cornwall. 

Fig.  117.  is  a  sketch  of  a  group  of  granite  veins  in  Cornwall, 
given  by  Messrs.  Von  Oeynhausen  and  Von  Dechen.f  The 
main  body  of  the  granite  here  is  of  a  porphyritic  appearance, 
with  large  crystals  of  felspar ;  but  in  the  veins  it  is  fine-grained, 
and  without  these  large  crystals.  The  general  height  of  the 

veins  is  from  six- 
Fig.  118.         ,,////////////!&  teen  to  twenty  feet, 

but  some  are  much 
higher. 

In  the  Valorsine, 
a  valley  not  far 
from  Mont  Blanc, 
in  Switzerland,  an 
ordinary  granite, 
consisting  of  fel- 

reins  of  granite  talcose  gneiss.    (L.  A.  Necker.)  sparj     quartZ)     and 

mica,  sends  forth  veins  into  a  talcose  gneiss,  (or  stratified  proto- 
gine,)  and  in  some  places  lateral  ramifications  are  thrown  off 
from  the  principal  veins  at  right  angles  (see  Fig.  118.)  the  veins, 
especially  the  minuter  ones,  being  finer  grained  than  the  granite 
in  mass. 

It  is  here  remarked,  that  the  schist  and  granite,  as  they  approach, 
seem  to  exercise  a  reciprocal  influence  on  each  other,  for  both 


*  On  Geol.  of  Cornwall,  Trans,  of  Cambridge  Soc.  vol.  i.  p.  124. 
t  Phil.  Mag.  and  Annals,  No.  27.  New  Series,  March,  1829. 


128 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Metals  near  Granite Isolated  Masses  of  Granite. 

undergo  a  modification  of  mineral  character.  The  granite,  still 
remaining  unstratified,  becomes  charged  with  green  particles ; 
and  the  talcose  gneiss  assumes  a  granitiform  structure,  without 
losing  its  stratification.* 

Professor  Keilhau  drew  my  attention  to  several  localities  in  the 
country  near  Christiania,  where  the  mineral  character  of  gneiss 
appears  to  have  been  affected  by  a  granite  of  much  newer  origin, 
for  some  distance  from  the  point  of  contact.  The  gneiss,  with- 
out losing  its  laminated  structure,  seems  to  have  become  charged 
with  a  larger  quantity  of  felspar,  and  that  of  a  redder  colour, 
than  the  felspar  usually  belonging  to  the  gneiss  of  Norway. 

Granite,  syenite,  and  those  porphyries  which  have  a  graniti- 
form structure,  in  short  all  plutonic  rocks,  are  frequently  observed 
to  contain  metals,  at  or  near  their  junction  with  stratified  forma- 
tions. On  the  other  hand,  the  veins  which  traverse  stratified 
rocks  are,  as  a  general  law,  more  metalliferous  near  such  junc- 
tions than  in  other  positions.  Hence  it  has  been  inferred  that 
these  metals  may  have  been  spread  in  a  gaseous  form  through 
the  fused  mass,  and  that  the  contact  of  another  rock,  in  a 
different  state  of  temperature,  or  sometimes  the  existence  of  rents 
in  other  rocks  in  the  vicinity,  may  have  caused  the  sublimation 
of  the  metals.f 

There  are  many  instances,  as  at  Markerud,  near  Christiania, 
in  Norway,  where  the  strike  of  the  beds  has  not  been  deranged 
throughout  a  large  area  by  the  intrusion  of  granite,  both  in  large 
masses  and  in  veins.  This  fact  is  considered  by  some  geologists 
to  militate  against  the  theory  of  the  forcible  injection  of  granite  in  a 

fluid  state.    But  it  may 
Fig.  119.  be  stated  in  reply,  that 

ramifying  dikes  of 
trap,  which  almost  all 
now  admit  to  have 
been  once  fluid,  pass 
through  the  same  fos- 
siliferous  strata,  near 
Christiania,  without 
deranging  their  strike 

ralorsine.    (L.  A.  Necker.)  or  dip.} 

The  real  or  apparent  isolation  of  large  or  small  masses  of 
granite  detached  from  the  main  body,  as  at  ab,  Fig.  119.,  and 


General  vieio  of  junction  of  granite  and  schist  oftkc 
(L. 


*  Necker,  sur  la  Val.  de  Valorsine,  Mem.  de  la  Soc.  de  Phys.  de  Geneve,  1828. 
—I  visited,  in  1832,  the  spot  referred  to  in  Fig.  118. 
t  Necker,  Proceedings  of  Geol.  Soc.,  No.  26.  p.  392. 
I  See  Keilhau's  Gaea  Norvegica ;  Christiania,  1838. 


PART  I.  CHAPTER  IX. 


129 


Fig.  120. 


gneiss. 


gneiss. 


quartz  Veins Conformable  Porphyries. 

above,  Fig.  113.,  and  a,  Fig.  118.,  has  been  thought  by  some 
writers  to  be  irreconcilable  with  the  doctrine  usually  taught  re- 
specting veins ;  but  many  of  them  may,  in  fact,  be  sections  of 
root-shaped  prolongations  of  granite  ;  while,  in  other  cases,  they 
may  in  reality  be  detached  portions  of  rock  having  the  plutonic 
structure.  For  there  may  have  been  spots  in  the  midst  of  the 
invaded  strata,  in  which  there  was  an  assemblage  of  materials 
more  fusible  than  the  rest,  or  more  fitted  to  combine  readily  into 
some  form  of  granite. 

Veins  of  pure  quartz  are  often  found  in  granite,  as  in  many 
stratified  rocks,  but  they  are  not  traceable,  like  veins  of  granite 
or  trap,  to  large  bodies  of  rock  of  similar  composition.  They 
appear  to  have  been  cracks,  into  which  siliceous  matter  was  infil- 
tered.  Such  segregation,  as  it  is  called,  can  sometimes  be  shown 
to  have  clearly  taken  place  long  subsequently  to  the  original  con- 
solidation of  the  containing  rock.  .Thus,  for  example,  in  the 
gneiss  of  Tronstad  Strand,  near  Drammen,  in  Norway,  the  an- 
nexed section  is  seen  on  the  beach.  It  appears  that  the  alter- 
nating strata  of  whitish 
granitiform  gneiss,  and 
black  hornblende-schist, 
were  first  cut  through  by 
a  greenstone  dike,  about 
2  ^  feet  wide;  then  the 
crack  a  b  passed  through 
all  these  rocks,  and  was 
filled  up  with  quartz.  The 
opposite  walls  of  the  vein 
are  in  some  parts  incrust- 
ed  with  transparent  crys- 
tals of  quartz,  the  middle 
of  the  vein  being  filled  up 
with  common  opaque  white  quartz. 

We  have  seen  that  the  volcanic  formations  have  been  called 
overlying,  because  they  not  only  penetrate  others,  but  spread 
over  them.  Mr.  Necker  has  proposed  to  call  the  granites  the 
underlying  igneous  rocks,  and  the  distinction  here  indicated  is 
highly  characteristic.  It  was  indeed  supposed  by  Von  Buch,  at 
the  commencement  of  his  geological  career,  that  the  granite  of 
Christiania,  in  Norway,  was  sometimes  intercalated  in  mountain 
masses  between  the  transition  strata  of  that  country,  overlying 
fossiliferous  shale  and  limestone.  But  although  the  granite  sends 
veins  into  these  fossiliferous  rocks,  and  is  decidedly  posterior  in 
origin,  the  opinion  expressed  of  its  actual  superposition  in  mass 
has  been  disproved  by  Professor  Keilhau,  some  of  whose  ob- 


a.  b.  Quartz  vein  passing  through  gneiss  and  green- 
stone^ Tronstad  Strand,  near  Christiania. 


130 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Conformable  Porphyries  .' Granitic  Rocks. 

servations  respecting  localities  referred  to  by  Von  Buch,  I  have 
lately  had  opportunities  of  verifying.  There  are,  however,  on 
a  smaller  scale,  certain  beds  of  euritic  porphyry >  some  a  few 
feet,  others  many  yards  in  thickness,  which  pass  into  granite, 
and  deserve  perhaps  to  be  classed  as  plutonic  rather  than  trap- 
pean  rocks,  which  may  truly  be  described  as  interposed  con- 
formably between  fossiliferous  strata,  as  the  porphyries  (a  c, 
Fig.  121.),  which  divide  the  bituminous  shales  and  argillaceous 
limestones,  ff.  But  some  of  these  same  porphyries  are  par- 
Fig.  121. 


Euritic  porphyry  alternating  with  fossiliferous  transition  strata,  near*Christiania. 

tially  unconformable,  as  &,  and  may  lead  us  to  suspect  that  the 
others  also,  notwithstanding  their  appearance  of  interstratifica- 
tion,  have  been  forcibly  injected.  Some  of  the  porphyritic  rocks 
above  mentioned  are  highly  quartzose,  others  very  felspathic. 
In  proportion  as  the  masses  are  more  voluminous,  they  become 
more  granitic  in  their  texture,  less  conformable,  and  even  begin 
to  send  forth  veins  into  contiguous  strata.  In  a  word,  we  have 
here  a  beautiful  illustration  of  the  intermediate  gradations  be- 
tween volcanic  and  plutonic  rocks,  not  only  in  their  mineralogical 
composition  and  structure,  but  also  in  their  relations  of  position 
to  associated  formations.  If  the  term  overlying  can  in  this  in- 
stance be  applied  to  a  plutonic  rock,  it  is  cnly  in  proportion  as 
that  rock  begins  to  acquire  a  trappean  aspect. 

It  has  been  already  hinted  that  the  heat,  which  in  every  ac- 
tive volcano  extends  downwards  to  indefinite  depths,  must  pro- 
duce simultaneously  very  different  effects  near  the  surface,  and 
far  below  it ;  and  we  cannot  suppose  that  rocks  resulting  from 
the  crystallizing  of  fused  matter  under  a  pressure  of  several 
miles  of  the  earth's  crust  can  resemble  those  formed  at  or  near 
the  surface.  Hence  the  production  at  great  depths  of  a  class 
of  rocks  analogous  to  the  volcanic,  and  yet  differing  m  many 
particulars,  might  almost  have  been  predicted,  even  had  we  no 
plutonic  formations  to  account  for.  How  well  these  agree,  both 
in  their  positive  and  negative  characters  with  the  theory  of  their 
deep  subterranean  origin,  the  student  will  be  able  to  judge  by 
considering  the  descriptions  already  given. 

It  has,  however,  been  objected,  that  if  the  granitic  and  vol- 


PART  I.     CHAPTER  X.  131 

Granite Metamorphic  Rocks. 

canic  rocks  were  simply'different  parts  of  one  great  series,  we 
ought  to  find  in  mountain  chains  volcanic  dikes  passing  upwards 
into  lava,  and  downwards  into  granite.  But  we  may  answer, 
that  our  vertical  sections  are  usually  of  small  extent ;  and  if  we 
find  in  certain  places  a  transition  from  trap  to  porous  lava,  and 
in  others  a  passage  from  granite  to  trap,  it  is  as  much  as  could 
be  expected  of  this  evidence. 

The  prodigious  extent  of  denudation  which  has  been  already 
demonstrated  to  have  occurred  at  former  periods,  will  reconcile 
the  student  to  the  belief,  that  crystalline  rocks  of  high  antiquity, 
although  deep  in  the  earth's  crust  when  originally  formed,  may 
have  become  uncovered  and  exposed  at  the  surface.  Their  ac- 
tual elevation  above  the  sea  may  be  referred  to  the  same  causes 
to  which  we  have  attributed  the  upheaval  of  marine  strata,  even 
to  the  summits  of  some  mountain  chains.  But  to  these  and 
other  topics,  I  shall  revert  when  speaking,  in  the  second  part, 
of  the  relative  ages  of  different  masses  of  granite. 


CHAPTER  X. 

METAMORPHIC   ROCKS. 

General  character  of  metamorphic  rocks — Gneiss — Hornblende-schist — Mica- 
schist — Clay-slate — Quarlzite — Chlorite-schist — Metamorphic  limestone — Alpha- 
betical list  and  explanation  of  other  rocks  of  this  family — Origin  of  the  meta- 
morphic strata — Their  stratification  is  real  and  distinct  from  cleavage — On  joints 
and  slaty  cleavage — Supposed  causes  of  these  structures — how  far  connected 
with  crystalline  action. 

WE  have  now  considered  three  distinct  classes  of  rocks  :  first, 
the  aqueous,  or  fossiliferous ;  secondly,  the  volcanic;  and, 
thirdly,  the  plutonic,  or  granitic ;  and  we  have  now  lastly  to  exa- 
mine those  crystalline  strata  to  which  the  name  of  metamorphic 
has  been  assigned.  The  last-mentioned  term  expresses,  as  before 
explained,  a  theoretical  opinion  that  such  strata,  after  having 
been  dejBsited  from  water,  acquired  by  the  influence  of  heat  and 
other  causes  a  highly  crystalline  texture. 

These  rocks,  when  in  their  most  characteristic  or  normal  state, 
are  wholly  devoid  of  organic  remains,  and  contain  no  distinct 
fragments  of  other  rocks  whether  rounded  or  angular.  They 
sometimes  break  out  in  the  central  parts  of  narrow  mountain 
chains,  but  in  other  cases  extend  over  areas  of  vast  dimensions, 


132      LYELL'S  ELEMENTS  OF  GEOLOGY. 


Metamorphic  Rocks Gneiss. 


occupying,  for  example,  nearly  the  whole  of  Norway  and  Swe- 
den, where,  as  in  Brazil,  they  appear  alike  in  the  lower  and 
higher  grounds.  In  Great  Britain  those  members  of  the  series 
which  approach  most  nearly  to  granite  in  their  composition* as 
gneiss,  mica-schist  and  hornblende-schist,  are  confined  to  the 
country  north  of  the  rivers  Forth  and  Clyde. 

Many  attempts  have  been  made  to  trace  a  general  order  of 
succession  or  superposition  in  the  members  of  this  family ;  gneiss, 
for  example,  having  been  often  supposed  to  hold  invariably  a 
lower  geological  position  than  mica-schist.  But  although  such 
an  order  may  prevail  throughout  limited  districts,  it  is  by  no 
means  universal,  nor  even  general  throughout  the  globe.  To 
this  subject,  however,  we  shall  again  revert  in  the  second  part  of 
this  volume,  when  the  chronological  relations  of  the  metamor- 
phic  rocks  are  pointed  out. 

The  following  may  be  enumerated  as  the  principal  members 
of  the  metamorphic  class,  gneiss,  mica-schist,  hornblende-schist, 
clay-slate,  chlorite-schist,  hypogene  or  metamorphic  limestone, 
and  certain  kinds  of  quartz  rock  or  quartzite. 

Gneiss. — The  first  of  these,  gneiss,  may  be  called  stratified 
granite,  being  formed  of  the  same  materials  as  granite,  namely 
felspar,  quartz,  and  mica.  In  the  specimen  here  figured,  the 

Fig.  122. 


Fragment  of  gneiss,  natural  size,  section  at  right  angles  to  planes 
of  stratification. 

white  layers  consist  almost  exclusively  of  granular  felspar,  with 
here  and  there  a  speck  of  mica  and  grain  of  quartz.  The  dark 
layers  are  composed  of  grey  quartz  and  black  mica^vith  oc- 
casionally a  grain  of  felspar  intermixed.  The  rock  sJR.ts  most 
easily  in  the  plane  of  these  darker  layers,  and  the  surface  thus 
exposed  is  almost  entirely  covered  with  shining  spangles  of  mica. 
The  accompanying  quartz  however  greatly  predominates  in 
quantity,  but  the  most  ready  cleavage  is  determined  by  the 
abundance  of  mica  in  certain  parts  of  the  dark  layer. 

Instead   of  these  thin  laminae,  gneiss  is   sometimes   simply 


PART  I.     CHAPTER  X.  133 

Hornblende-Schist Mica-Schist Clay-Slate Quartzite. 

divided  into  thick  beds,  in  which  the  mica  has  only  a  slight  degree 
of  parallelism  to  the  planes  of  stratification. 

The  term  "  gneiss,"  however,  in  geology  is  commonly  used  in 
a  wider  sense  to  designate  a  formation  in  which  the  above  men- 
tioned rock  prevails,  but  with  which  any  one  of  the  other  meta- 
morphic  rocks,  and  more  especially  hornblende-schist,  may  alter- 
nate. These  other  members  of  the  metamorphic  series  are,  in 
this  case,  considered  as  subordinate  to  the  true  gneiss.  In  some 
rare  instances  fragments  of  pre-existing  rocks  may  be  detected 
in  gneiss. 

The  different  varieties  of  rock  allied  to  gneiss,  into  which  fel- 
spar enters  as  an  essential  ingredient,  will  be  understood  by  re- 
ferring to  what  was  said  of  granite.  Thus,  for  example,  horn- 
blende may  be  superadded  to  mica,  quartz,  and  felspar,  forming 
a  syenitic  gneiss ;  or  talc  may  be  substituted  for  mica,  consti- 
tuting talcose  gneiss,  a  rock  composed  of  felspar,  quartz,  and 
talc,  in  distinct  crystals  or  grains  (stratified  protogine  of  the 
French.) 

Hornblende-schist  is  usually  black,  and  composed  principally 
of  hornblende,  with  a  variable  quantity  of  felspar,  and  some- 
times grains  of  quartz.  When  the  hornblende  and  felspar  are 
nearly  -in  equal  quantities,  and  the  rock  is  not  slaty,  it  corre- 
sponds in  character  with  the  greenstones  of  the  trap  family,  and 
has  been  called  "  primitive  greenstone."  Some  of  these  horn- 
blendic  masses  may  really  have  been  volcanic  rocks,  which 
have  since  assumed  a  more  crystalline  or  metamorphic  texture. 

Mica^schist,  or  micaceous  schist,  is,  next  to  gneiss,  one  of  the 
most  abundant  rocks  of  the  metamorphic  series.  It  is  slaty, 
essentially  composed  of  mica  and  quartz,  the  mica  sometimes 
appearing  to  constitute  the  whole  mass.  Beds  of  pure  quartz 
also  occur  in  this  formation.  In  some  districts  garnets  in  regu- 
lar twelve-sided  crystals  form  an  integrant  part  of  mica-schist. 
This  rock  passes  by  insensible  gradations  into  clay-slate. 

Clay-slate,  or  Argillaceous  schist. — This  rock  resembles  an 
indurated  clay  or  shale,  is  for  the  most  part  extremely  fissile, 
often  affording  good  roofing  slate.  It  may  consist  of  the  ingre- 
dients of  gneiss,  or  of  an  extremely  fine  mixture  of  mica  and 
quartz,  or  talc  and  quartz.  Occasionally  it  derives  a  shining 
and  sill^  lustre  from  the  minute  particles  of  mica  or  talc  which 
it  contains.  It  varies  from  greenish  or  bluish-grey  to  a  lead 
colour.  It  may  be  said  of  this,  more  than  of  any  other  schist, 
that  it  is  common  to  the  metamorphic  and  fossiliferous  series, 
for  some  clay-slates  taken  from  each  division  would  not  be  dis- 
tinguishable by  mineralogical  characters. 

Quartzite,   or   Quartz   rock,  is  an   aggregate  of  grains  of 

M 


134      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Chlorite-schist Hypogene  Limestone Metamorphic  Rocks. 

quartz,  which  are  either  in  minute  crystals,  or  in  many  cases 
slightly  rounded,  occurring  in  regular  strata  associated  with 
gneiss  or  other  metamorphic  rocks.  Compact  quartz,  like  that  so 
frequently  found  in  veins,  is  also  found  together  with  granular 
quartzite.  Both  of  these  alternate  with  gneiss  or  mica-schist,  or 
pass  into  those  rocks  by  the  addition  of  mica,  or  of  felspar  and 
mica. 

Chlorite-schist  is  a  green  slaty  rock  in  which  chlorite  is 
abundant  in  foliated  plates,  usually  blended  with  minute  grains 
of  quartz,  or  sometimes  with  felspar  or  mica.  Often  associated 
with,  and  graduating  into,  gneiss  and  clay-slate. 

Hypogene  or  metamorphic  limestone. — This  rock,  commonly 
called  primary  limestone,  is  sometimes  a  thick  bedded  white 
crystalline  granular  marble  used  in  sculpture ;  but  more  fre- 
quently it  occurs  in  thin  beds,  forming  a  foliated  schist  much 
resembling  in  colour  and  appearance  certain  varieties  of  gneiss 
and  mica-schist.  It  alternates  with  both  these  rocks,  and  in  like 
manner  with  argillaceous  schist.  It  then  usually  contains  some 
crystals  of  mica,  and  occasionally  quartz,  felspar,  hornblende, 
and  talc.  This  member  of  the  metamorphic  series  enters 
sparingly  into  the  structure  of  the  hypogene  districts  of  Nor- 
way, Sweden,  and  Scotland,  but  is  largely  developed  in  the  Alps. 

Before  offering  any  further  observations  on  the  probable  origin 
of  the  metamorphic  rocks,  I  subjoin  in  the  form  of  a  glossary, 
a  brief  explanation  of  some  of  the  principal  varieties  and  their 
synonymes. 

ACTINOLJTE-SCHIST.  A  slaty  foliated  rock,  composed  chiefly  of  actinolite,  (an 
emerald-green  mineral,  allied  to  hornblende,)  with  some  admixture  of  fel- 
spar, or  quartz,  or  mica. 

AMPELITE.  Aluminous  slate  (Brongniart) ;  occurs  both  in  the  metamorphic  and 
fossiliferous  series. 

AMPHIBOLITE.    Hornblende  rock,  which  see. 

ARGILLACEOUS-SCHIST,  or  CLAY-SLATE.    See  p.  133. 

CHIASTOLITE-SLATE  scarcely  differs  from  clay-slate,  but  includes  numerous  crys- 
tals of  Chiastolite  ;  in  considerable  thickness  in  Cumberland.  Chiastolite 
occurs  in  long  slender  rhomboidai  crystals.  For  composition,  see  Table,  p. 
102. 

CHLORITE-SCHIST.  A  green  slaty  rock,  in  which  chlorite,  a  green  scaly  mineral, 
is  abundant  See  p.  134. 

CLAY-SLATE,  or  ARGILLACEOUS-SCHIST.    See  p.  133. 

EURITE  and  EURITIC  PORPHYRY.  A  base  of  compact  felspar,  with  grains  of 
laminar  felspar,  and  often  mica  and  other  minerals  disseminated  (Brong- 
niart.) M.  D'Aubuisson  regards  eurite  as  an  extremely  fine  grained  granite, 
in  which  felspar  predominates,  the  whole  forming  an  apparently  homogene- 
ous rock.  Eurite  has  been  already  mentioned  as  a  plutonic  rock,  but  occurs 
also  in  beds  subordinate  to  gneiss  or  mica-slate. 

GNEISS.    A  stratified  or  laminated  rock,  same  composition  as  granite.  See  p.  132. 


PART  I.  CHAPTER 

Metamorphic  Rocks,  and  their  Origin. 


HORNBLENDE  ROCK,  or  AMPHIBOLITE.  The  same  composition  as  hornblende- 
schist,  stratified,  but  not  fissile.  See  p.  100. 

HORNBLENDE-SCHIST,  or  SLATE.  Composed  chiefly  of  hornblende,  with  occa- 
sionally some  felspar.  See  p.  133. 

HORNBLENDIC  or  SvENiTic  GNEISS.  Composed  of  felspar,  quartz,  and  horn- 
blende. 

HYPOGENE  LIMESTONE.    See  p.  134. 

MARBLE.    See  p.  134. 

MICA-SCHIST,  or  MICACEOUS-SCHIST.  A  slaty  rock,  composed  of  mica  and  quartz 

in  variable  proportions.     See  p.  133. 
MICA-SLATE.    See  MICA-SCHIST,  p.  133. 

PHYLLADE.    D'Aubuisson's  term  for  clay-slate,  from  0uAAa?,  a  heap  of  leaves. 
PRIMARY  LIMESTONE.    See  HYPOGENE  LIMESTONE,  p.  134. 

PROTOGINE.  See  TALCOSE-GNEISS,  p.  133.:  when  unstratified  it  is  Talcose-gra- 
nite. 

QUARTZ  ROCK,  or  QUARTZITE.  A  stratified  rock ;  an  aggregate  of  grains  of 
quartz.  See  p.  133. 

SERPENTINE  occurs  in  both  divisions  of  the  hyppgene  series,  as  a  stratified  or 
unstratified  rock;  contains  much  magnesia;  is  chiefly  composed  of  the  min- 
eral called  serpentine,  mixed  with  diallage,  talc,  and  steatite.  The  pure 
varieties  of  this  rock,  called  noble  serpentine,  consist  of  a  hydrated  silicate 
of  magnesia,  generally  of  a  greenish  colour ;  this  base  is  commonly  mixed 
with  oxide  of  iron. 

TALCOSE-GNEISS.  Same  composition  as  talcose  granite  or  protogine,  but  either 
stratified  or  laminated. 

TALCOSE-SCHIST  consists  chiefly  of  talc,  or  of  talc  and  quartz,  or  of  talc  and  fel- 
spar, and  has  a  texture  something  like  that  of  clay-slate. 

WHITESTONE.    Same  as  Eurite. 

Origin  of  the  Metamorphic  Strata. 

Having  said  thus  much  of  the  mineral  composition  of  the  met- 
amorphic  rocks,  I  may  combine  what  remains  to  be  said  of  their 
structure  and  history,  with  an  account  of  the  opinions  entertained 
of  their  probable  origin.  At  the  same  time  it  may  be  well  to  fore- 
warn the  reader  that  we  are  here  entering  upon  ground  of  con- 
troversy, and  soon  reach  the  limits  where  positive  induction  ends, 
and  beyond  which  we  can  only  indulge  in  speculations.  It  was 
once  a  favourite  doctrine,  and  is  still  maintained  by  many,  that 
these  rocks  owe  their  crystalline  texture,  their  want  of  all  signs 
of  a  mechanical  origin,  or  of  fossil  contents,  to  a  peculiar  and 
nascent  condition  of  the  planet  at  the  period  of  their  formation. 
The  arguments  in  refutation  of  this  hypothesis  will  be  more  fully 
considered  when  I  show,  in  the  second  part  of  this  volume,  to 
how  many  different  ages  the  metamorphic  formations  are  refer- 
able, and  how  gneiss,  mica-schist,  clay-slate,  and  hypogene  lime- 
stone (that  of  Carrara  for  example,)  have  been  formed,  not  only 


136      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Origin  of  tlie  Metamorphic  Rocks. 

since  the  first  introduction  of  organic  beings  into  this  planet,  but 
even  long  after  many  distinct  races  of  plants  and  animals'  had 
passed  away  in  succession. 

The  doctrine  respecting  the  crystalline  strata,  implied  in  the 
name  metamorphic,  may  properly  be  treated  of  in  this  place ; 
and  we  must  first  inquire  whether  these  rocks  are  really  entitled 
to  be  called  stratified  in  the  strict  sense  of  having  been  origin- 
ally deposited  as  sediment  from  water.  The  general  adoption 
by  geologists  of  the  term  stratified,  as  applied  to  these  rocks, 
sufficiently  attests  their  division  into  beds  very  analogous,  at 
least  in  form,  to  ordinary  fossiliferous  strata.  This  resemblance 
is  by  no  means  confined  to  the  existence  in  both  of  an  .occa- 
sional slaty  structure,  but  extends  to  every  kind  of  arrangement 
which  is  compatible  with  the  absence  of  fossils,  and  of  sand, 
pebbles,  ripple  mark,  and  other  characters  which  the  meta- 
morphic theory  supposes  to  have  been  obliterated  by  plutonic 
action.  Thus,  for  example,  we  behold  alike  in  the  crystalline 
and  fossiliferous  formations  an  alternation  of  beds  varying 
greatly  in  composition,  colour,  and  thickness.  We  observe,  for 
instance,  gneiss  alternating  with  layers  of  black  hornblende- 
schist,  or  with  granular  quartz,  or  limestone;  and  the  inter- 
change of  these  different  strata  may  be  repeated  for  an  indefi- 
nite number  of  times.  In  the  like  manner,  mica-schist  alter- 
nates with  chlorite-schist,  and  with  granular  limestone  in  thin 
layers. 

As  in  fossiliferous  formations  strata  of  pure  siliceous  sand 
alternate  with  micaceous  sand  and  with  layers  of  clay,  so  in 
the  crystalline  or  metamorphic  rocks  we  have  beds  of  pure 
quartzite  alternating  with  mica-schist  and  clay-slate.  As  in  the 
secondary  and  tertiary  series  we  meet  with  limestone  alternating 
again  and  again  with  micaceous  or  argillaceous  sand,  so  we 
find  in  the  hypogene,  gneiss  and  mica-schist  alternating  with 
pure  and  impure  granular  limestones. 

It  has  also  been  shown  that  the  ripple  mark  is  very  com- 
monly repeated  throughout  a  considerable  thickness  of  fossil- 
iferous strata,  so  in  mica-schist  and  gneiss,  there  is  sometimes 
an  undulation  of  the  laminae  on  a  minute  scale,  which  may, 
perhaps,  be  a  modification  of  similar  inequalities  in  the  original 
deposit. 

In  the  crystalline  formations  also,  as  in  many  of  the  sedi- 
mentary before  described,  single  strata  are  sometimes  made  up 
of  laminae  placed  diagonally,  such  laminae  not  being  regularly 
parallel  to  the  planes  of  cleavage. 


PART  I.    CHAPTER  X;  137 

Slaty  Cleavage. 

Fig.  123.  ™s   disposition    of   the 

layers  is  illustrated  in  the 
accompanying  diagram,  in 
which  I  have  represented 
carefully  the  stratification 
of  a  coarse  argillaceous 
schist,  which  I  examined  in 
the  Pyrenees,  part  of  which 
approaches  in  character  to 
a  green  and  blue  roofing 

Lamination  of  clay-slate,  Montague  de  Seguinat,  slate,  while  part  is  extremely 
near  Gavarnie,  in  tke  Pyrenees.  quartzose,    the    whole    mass 

passing  downwards  into  micaceous  schist.  The  vertical  section 
here  exhibited  is  about  three  feet  in  height,  and  the  layers  are 
sometimes  so  thin  that  fifty  may  be  counted  in  the  thickness  of 
an  inch.  Some  of  them  consist  of  pure  quartz. 

The  inference  drawn  from  the  phenomena  above  described, 
in  favour  of  the  aqueous  origin  of  clay-slate  and  other  crys- 
talline strata,  is  greatly  strengthened  by  the  fact  that  many  of 
these  metamorphic  rocks  occasionally  alternate  with,  and  some- 
times pass,  by  intermediate  gradations,  into  rocks  of  a  decidedly 
mechanical  origin,  and  exhibiting  traces  of  organic  remains. 
The  fossiliferous  formations,  moreover,  into  which  this  passage 
is  effected,  are  by  no  means  invariably  of  the  same  age  nor  of 
the  highest  antiquity,  as  will  be  afterwards  explained.  (See 
Part  II.) 

Stratification  of  the  metamorphic  rocks  distinct  from  cleav- 
age.— The  beds  into  which  gneiss,  mica-schist,  and  hypogene 
limestone  divide,  exhibit  most  commonly,  like  ordinary  strata,  a 
want  of  perfect  geometrical  parallelism.  For  this  reason,  there- 
fore, in  addition  to  the  alternate  recurrence  of  layers  of  distinct 
materials,  the  stratified  arrangement  of  the  crystalline  rocks 
cannot  be  explained  away  by  supposing  it  to  be  simply  a  divi- 
sional structure  like  that  to  which  we  owe  some  of  the  slates 
used  for  writing  and  roofing.  Slaty  cleavage,  as  it  has  been 
called,  has  in  many  cases  been  produced  by  the  regular  depo- 
sition of  thin  plates  of  fine  sediment  one  upon  another ;  but 
there  are  many  instances  where  it  is  decidedly  unconnected  with 
such  a  mode  of  origin,  and  where  it  is  not  even  confined  to  the 
aqueous  formations.  Some  kinds  of  trap,  for  example,  as  clink- 
stone, split  into  laminae,  and  are  used  for  roofing. 

There  are,  says  Professor  Sedgwick,  three  distinct  forms  of 
structure  exhibited  in  certain  rocks  throughout  large  districts : 
viz. — First,  stratification;  secondly,  joints;  and  thirdly,  slaty 
cleavage ;  the  two  last  having  no  connection  with  true  bedding, 


138  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Jointed  Structure Cleavage  and  Joints. 

and  having  been  superinduced  by  causes  absolutely  independent 
of  gravitation.  All  these  different  structures  must  have  different 
names,  even  though  there  be  some  cases  where  it  is  impossible, 
after  carefully  studying  the  appearances,  to  decide  upon  the  class 
to  which  they  belong.* 

Joints. — Now  in  regard  to  the  second  of  these  forms  of 
structure  or  joints,  they  are  natural  fissures  which  often  tra- 
verse rocks  in  straight  and  well  determined  lines.  They  afford 
to  the  quarryman,  as  Mr.  Murchison  observes,  when  speaking 
of  the  phenomena,  as  exhibited  in  Shropshire  and  the  neigh- 
bouring counties,  the  greatest  aid  in  the  extraction  of  blocks  of 
stone,  and,  if  a  sufficient  number  cross  each  other,  the  wht>le 
mass  of  rock  is  split  into  symmetrical  blocks,  f  The  faces  of 
the  joints  are  for  the  most  part  smoother  and  more  regular  than 
the  surfaces  of  true  strata.  The  joints  are  straight-cut  chinks, 
often  slightly  open,  often  passing,  not  only  through  layers  of 
successive  deposition,  but  also  through  balls  of  limestone  or 
other  matter  which  have  been  formed  by  concretionary  action, 
since  the  original  accumulation  of  the  strata.  Such  joints, 
therefore,  must  often  have  resulted  from  one  of  the  last  changes 
superinduced  upon  sedimentary  deposits.:}: 

In  the  annexed  diagram  the  flat  surfaces  of  rock  A,  B,  C,  re- 
present exposed  faces  of  joints,  to  which  the  walls  of  other 
joints,  J  J,  are  parallel.  S  S  are  the  lines  of  stratification ;  C  C 
are  lines  of  slaty  cleavage,  which  intersect  the  rock  at  a  con- 
siderable angle  to  the  planes  of  stratification. 


J  J 

Stratification,  joints,  and  cleavage. 

Joints  according  to  Professor  Sedgwick  are  distinguishable 
from  lines  of  slaty  cleavage  in  this,  that  the  rock  intervening 

*  Geol.  Trans.,  Second  Series,  vol.  iii.  p.  480. 

t  The  Silurian  System  of  Rocks,  as  developed  in  Salop,  Hereford,  &c.,  p.  845. 

tlbid.  p.246. 


PART  I.    CHAPTER  X.  139 

Slaty  Cleavage. 

between  two  joints  has  no  tendency  to  cleave  in  a  direction 
parallel  to  the  planes  of  the  joints,  whereas  a  rock  is  capable 
of  indefinite  subdivision  in  the  direction  of  its  slaty  cleavage. 
In  some  cases  where  the  strata  are  curved,  the  planes  of  cleav- 
age are  still  perfectly  parallel.  This  has  been  observed  in  the 
slate  rocks  of  part  of  Wales  (See  Fig.  125.),  which  consist  of  a 

Fig.  125. 


Parallel  planes  of  cleavage  intersecting'  curved  strata.    (Sedgwick.) 

hard  greenish  slate.  The  true  bedding  is  there  indicated  by  a 
number  of  parallel  stripes,  some  of  a  lighter  and  some  of  a 
darker  colour  than  the  general  mass.  Such  stripes  are  found  to 
be  parallel  to  the  true  planes  of  stratification,  wherever  these 
are  manifested  by  ripple-mark,  or  by  beds  containing  peculiar 
organic  remains.  Some  of  the  contorted  strata  are  of  a  coarse 
mechanical  structure,  alternating  with  fine-grained  crystalline 
chloritic  slates,  in  which  case  the  same  slaty  cleavage  extends 
through  the  coarser  and  finer  beds,  though  it.  is  brought  out  in 
greater  perfection  in  proportion  as  the  materials  of  the  rock  are 
fine  and  homogeneous.  It  is  only  when  these  are  very  coarse 
that  the  cleavage  planes  entirely  vanish.  These  planes  are  usu- 
ally inclined  at-  a  very  considerable  angle  to  the  planes  of  the 
strata.  In  the  Welsh  chains,  for  example,  the  average  angle  is 
as  much  as  from  30°  to  40°.  Sometimes  the  cleavage  planes 
clip  towards  the  same  point  of  the  compass  as  those  of  stratifi- 
cation, but  more  frequently  to  opposite  points.  It  may  be  stated 
as  a  general  rule,  that  when  beds  of  coarser  materials  alternate 
with  those  composed  of  finer  particles,  the  slaty  cleavage  is 
either  entirely  confined  to  the  fine-grained  rock,  or  is  very  im- 
perfectly exhibited  in  that  of  coarser  texture.  This  rule  holds, 
whether  the  cleavage  is  parallel  to  the  planes  of  stratification 
or  not. 

In  the  Swiss  and  Savoy  Alps,  as  Mr.  Bakewell  has  remarked, 
enormous  masses  of  limestone  are  cut  through  so  regularly  by 
nearly  vertical  partings,  and  these  are  often  so  much  more  con- 
spicuous than  the  seams  of  stratification,  that  an  unexperienced 
observer  will  almost  inevitably  confound  them,  and  suppose  the 
strata  to  be  perpendicular  in  places  where  in  fact  they  are  almost 
horizontal.* 

*  Introduction  to  Geology,  chap.  iv. 


140      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Jointed  Structure  in  Rocks. 

Now  these  joints  are  supposed  to  be  analogous  to  those  part- 
ings which  have  been  already  observed  to  separate  volcanic  and 
plutonic  rocks  into  cuboidal  and  prismatic  masses.  On  a  small 
scale  we  see  clay  and  starch  when  dry  split  into  similar  shapes, 
which  is  often  caused  by  simple  contraction,  whether  the  shrink- 
ing be  due  to  the  evaporation  of  water,  or  to  a  change  of  tem- 
perature. It  is  well  known  that  many  sandstones  and  other 
rocks  expand  by  the  application  of  moderate  degrees  of  heat, 
and  then  contract  again  on  cooling ;  and  there  can  be  no  doubt 
that  large  portions  of  the  earth's  crust  have,  in  the  course  of 
past  ages,  been  subjected  again  and  again  to  very  different  de- 
grees of  heat  and  cold.  These  alternations  of  temperature  have 
probably  contributed  largely  to  the  production  of  joints  in 
rocks. 

In  some  countries,  as  in  Saxony,  where  masses  of  basalt  rest 
on  sandstone,  the  aqueous  rock  has  for  the  distance  of  several 
feet  from  the  point  of  junction  assumed  a  columnar  structure  sim- 
ilar to  that  of  the  trap.  In  like  manner  some  hearthstones,  after 
exposure  to  the  heat  of  a  furnace  without  being  melted,  have  be- 
come prismatic.  Certain  crystals  also  acquire  by  the  application 
of  heat  a  new  internal  arrangement,  so  as  to  break  in  a  new  di- 
rection, their  external  form  remaining  unaltered. 

Scoresby,  when  speaking  of  the  icebergs  of  Spitzbergen,  states 
that  "  they  are  full  of  rents,  extending  perpendicularly  down- 
wards, and  dividing  them  into  innumerable  columns."  Colonel 
Jackson,  who  has  lately  investigated  this  subject  more  attentively, 
found  that  the  ice  on  the  Neva,  at  St.  Petersburg,  at  the  begin- 
ning of  a  thaw,  when  two  feet  in  thickness,  is  traversed  by  rows 
of  very  minute  air-bubbles  extending  in  straight  lines,  sometimes 
a  little  inflected,  from  the  upper  surface  of  the  ice  towards  the 
lower,  within  from  two  to  five  inches  of  which  they  terminate. 
'*  Other  blocks  presented  these  bubbles  united,  so  as  to  form  cy- 
lindrical canals,  a  little  thicker  than  a  horsehair.  Observing  still 
further,"  he  says,  "  I  found  blocks  in  which  the  process  was 
more  advanced,  and  two,  three,  or  more  clefts,  struck  off  in  dif- 
ferent directions  from  the  vertical  veins,  so  that  a  section  perpen- 
dicular to  the  vein  would  represent  in  miniature  the  star-formed 
cracks  of  timber.  Finally,  in  some  pieces,  these  cracks  united 
from  top  to  bottom  of  the  veins,  separating  the  whole  mass  into 
vertical  prisms,  having  a  greater  or  less  number  of  sides.  In 
this  state  a  slight  shock  was  sufficient  to  detach  them  ;  and  the 
block  with  its  scattered  fragments  was  in  all  respects  the  exact 
miniature  resemblance,  in  crystal,  of  a  Giant's  Causeway.  The 
surface  was  like  a  tessellated  pavement,  and  the  columns  rose 
close,  adhering  and  parallel,  from  the  compact  mass  of  a  few 


PART  I.     CHAPTER  X.  141 

Jointed  Structure  and  Cleavage. 

inches  at  the  under  surface.  More  or  less  -time  is  required  for 
the  process,  which  I  have  since  seen  in  all  its  different  stages."* 
Here  again  we  find  the  columnar  or  jointed  structure  in  a  solid 
mass,  which  had  been  subjected  to  great  changes  of  temperature. 
It  seems,  therefore,  that  the  fissures  called  joints  may  have 
been  the  result  of  different  causes,  as  of  some  modification  of 
crystalline  action,  or  simple  contraction  during  consolidation,  or 
during  a  change  of  temperature.  And  there  are  cases  where 
joints  may  have  been  due  to  mechanical  violence,  and  the  strain 
exerted  on  strataMuring  their  upheaval,  or  when  they  have  sunk 
down  below  their  former  level.  Professor  Phillips  has  suggested 
that  the  previous  existence  of  divisional  planes  may  often  have 
determined,  and  must  greatly  have  modified,  the  lines  and  points 
of  fracture  caused  in  rocks  by  those  forces  to  which  they  owe 
their  elevation  or  dislocations.  These  lines  and  points  being 
those  of  least  resistance,  cannot  fail  to  have  influenced  the  direc- 
tion in  which  the  solid  mass  would  give  way  on  the  application 
of  external  force. 

It  has  been  observed  by  Mr.  Murchison,  that  in  referring  both 
joints  and  slaty  cleavage  to  crystalline  action,  we  are  borne  out 
by  a  well-known  analogy  in  which  crystallization  has  in  like 
'manner  given  rise  to  two  distinct  kinds  of  structure  in  the  same 
body.  Thus  for  example,  in  a  six-sided  prism  of  quartz,  the 
planes  of  cleavage  are  distinct  from  those ~of  the  prism.  It  is 
impossible  to  cleave  the  crystals  parallel  to  the  plane  of  the  prism, 
just  as  slaty  rocks  cannot  be  cleaved  parallel  to  the  joints,  but 
the  quartz  crystal,  like  the  older  schists,  may  be  cleaved  ad  infi- 
nitum  in  the  direction  of  the  cleavage  planes.f 

I  have  already  stated  that  extremely  fine  slates,  like  those  of 
the  Niesen,  near  the  Lake  of  Thun,  in  Switzerland,  are  perfectly 
parallel  to  the  planes  of  stratification,  and  are,  therefore,  proba- 
bly due  to  successive  aqueous  deposition.  Even  when  the  slates 
are  oblique  to  the  general  planes  of  the  strata,  it  by  no  means 
follows  as  a  matter  of  course,  that  they  have  been  caused  by 
crystalline  action,  for  they  may  be  the  result  of  that  diagonal 
lamination  which  I  have  before  described  (p.  32.)  In  this  case, 
however,  there  is  usually  much  irregularity,  whereas  those  clea- 
vage planes  oblique  to  the  true  stratification,  which  are  referred 
to  a  crystalline  action,  are  often  perfectly  symmetrical,  and  ob- 
serve a  strict  geometrical  parallelism,  even  when  the  strata  are 
contorted,  as  already  described  (p.  139.) 

In  regard  to  the  origin  of  slaty  cleavage,  where  it  is  uncon- 

*  Journ.  of  Roy.  Geograph.  Soc.,  vol.  v.  p.  19. 
t  Silurian  System  of  Rocks,  &c.,  p.  246. 


142  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Slaty  Cleavage. 

nected  with  sedimentary  deposition,  Professor  Sedgwick  is  of 
opinion  that  no  retreat  of  parts,  no  contraction  in  dimensions,  in 
passing  to  a  solid  state,  can  account  for  the  phenomenon.  It 
must  be  referred  to  crystalline  or  polar  forces  acting  simultane- 
ously and  somewhat  uniformly,  in  given  directions,  on  large 
masses  having  a  homogeneous  composition. 

A  fact  recorded  by  Mr.  Darwin  affords  confirmation  to  this 
theory.  The  ore  of  the  gold  mines  of  Yaquil,  in  Chili,  is  ground 
in  a  mill  into  an  impalpable  powder.  After  this  powder  has  Been 
washed,  and  nearly  all  the  metal  separated,  the  mud  which  passes 
from  the  mills  is  collected  into  pools,  where  it  subsides,  and  is 
cleared  out  and  thrown  into  a  common  heap.  A  great  deal  of 
chemical  action  then  commences,  salts  of  various  kinds  effloresce 
on  the  surface,  and  the  mass  becomes  hard,  and  divides  into  con- 
cretionary fragments.  These  fragments  were  observed  to  pos- 
sess an  even  and  well  defined  slaty  structure  ;  but  the  laminae 
were  not  inclined  at  any  uniform  angle.* 

Mr.  R.  W.  Fox  lately  submitted  a  mass  of  moist  clay,  worked 
up  with  acidulated  water,  to  weak  voltaic  action  for  some  months, 
and  it  was  found  when  dry  to  be  rudely  laminated,  the  planes  of 
the  slightly  undulating  laminae  being  at  right  angles  to  the  direc- 
tion of  the  electrical  forces.f 

Sir  John  Herschel,  in  allusion  to  slaty  cleavage,  has  suggested, 
"  that  if  rocks  have  been  so  heated  as  to  allow  a  commencement 
of  crystallization ;  that  is  to  say,  if  they  have  been  heated  to  a 
point  at  which  the  particles  can  begin  to  move  amongst  them- 
selves, or  at  least  on  their  own  axes,  some  general  law  must  then 
determine  the  position  in  which  these  particles  will  rest  on  cool- 
ing. Probably,  that  position  will  have  some  relation  to  the  di- 
rection in  which  the  heat  escapes.  Now,  when  all,  or  a  majority 
of  particles  of  the  same  nature  have  a  general  tendency  to  one 
position,  that  must  of  course  determine  a  cleavage  plane.  Thus 
we  see  the  infinitesimal  crystals  of  fresh  precipitated  sulphate  of 
baryte,  and  some  other  such  bodies,  arrange  themselves  alike  in 
the  fluid  in  which  they  float ;  so  as,  when  stirred,  all  to  glance 
with  one  light,  and  give  the  appearance  of  silky  filaments.  Some 
sorts  of  soap,  in  which  insoluble  margarates:}:  exist,  exhibit  the 
same  phenomenon  when  mixed  with  water ;  and  what  occurs  in 
our  experiments  on  a  minute  scale  may  occur  in  nature  on  a 
great  one."§ 

*  Journal,  p.  324. 

t  Although  the  lamination  in  the  specimen  shown  to  me  was  very  imperfect, 
it  was  sufficiently  evident  to  encourage  farther  experiments. 

t  Margaric  acid  is  an  oleaginous  acid,  formed  from  different  animal  and  vege- 
table fatty  substances.  A  margarate  is  a  compound  of  this  acid  with  soda,  pot- 
ash, or  some  other  base,  and  is  so  named  from  its  pearly  lustre. 

$  Letter  to  the  author,  dated  Cape  of  Good  Hope,  Feb.  20,  1836. 


PART  I.     CHAPTER  XL  143 


Alterations  of  Strata  in  contact  with  Granite. 


CHAPTER  XL 

METAMORPHIC  ROCKS  —  continued. 

Strata  near  some  intrusive  masses  of  granite  converted  into  rocks  identical 
with  different  members  of  the  metamorphic  series — Arguments  hence  derived  as 
to  the  nature  of  plutonic  action — Time  may  enable  this  action  to  pervade  denser 
masses — From  what  kinds  of  sedimentary  rock  each  variety  of  the  metamorphic 
class  may  be  derived — Certain  objections  to  the  metamorphic  theory  considered. 

IT  has  been  seen  that  geologists  have  been  very  generally  led 
to  infer,  from  the  phenomena  of  joints  and  slaty  cleavage,  that 
mountain  masses,  of  which  the  sedimentary  origin  is  unquestion- 
able, have  been  acted  upon  simultaneously  by  vast  crystalline 
forces.  That  the  structure  of  fossiliferous  strata  has  often  been 
modified  by  some  general  cause  since  their  original  deposition, 
and  even  subsequently  to  their  consolidation  and  dislocation,  is 
undeniable.  These  facts  prepare  us  to  believe,  that  still  greater 
changes  may  have  been  worked  out  by  a  greater  intensity,  or 
more  prolonged  development  of  the  same  agency,  combined,  per- 
haps, with  other  causes.  Now  we  have  seen  that,  near  the  im- 
mediate contact  of  granite  veins  and  volcanic  dikes,  very  extra- 
ordinary alterations  in  rocks  have  taken  place,  more  especially 
in  the  neighbourhood  of  granite.  It  will  be  useful  here  to  add 
other  illustrations,  showing  that  a  texture  undistinguishable  from 
that  which  characterizes  the  more  crystalline  metamorphic  form- 
ations, has  actually  been  superinduced  in  strata  once  fossilife- 
rous. 

In  the  southern  extremity  of  Norway,  there  is  a  large  district, 
on  the  west  side  of  the  fiord  of  Christiana,  in  which  granite  or 
syenite  protrudes  in  mountain  masses  through  fossiliferous  strata, 
and  usually  sends  veins  into  them  at  the  point  of  contact.  The 
stratified  rocks,  replete  with  shells  and  zoophytes,  consist  chiefly 
of  shale,  limestone,  and  some  sandstone,  and  all  these  are  inva- 
riably altered  near  the  granite  for  a  distance  of  from  50  to  400 
yards.  The  aluminous  shale  are  hardened  and  have  become 
flinty.  Sometimes  they  resemble  jasper.  Ribboned  jasper  is 
produced  by  the  hardening  of  alternate  layers  of  green  and  cho- 
colate-coloured schist,  each  stripe  faithfully  representing  the  ori- 
ginal lines  of  stratification.  Nearer  the  granite  the  schist  often 
contains  crystals  of  hornblende,  which  are  even  met  with  in  seme 


144 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Alterations  of  Strata  in  contact  with  Granite. 


places  for  a  distance  of  several  hundred  yards  from  the  junction ; 
and  this  black  hornblende  is  so  abundant,  that  eminent  geologists, 
when  passing  through  the  country,  have  confounded  it  with  the 
ancient  hornblende-schist,  subordinate  to  the  great  gneiss  forma- 
tion of  Norway.  Frequently,  between  the  granite  and  the  horn- 
blendic  slate,  above  mentioned,  grains  of  mica  and  crystalline 
felspar  appear  in  the  schist,  so  that  rocks  resembling  gneiss  and 
mica-schist  are  produced.  Fossils  can  rarely  be  detected  in  these 
schists,  and  they  are  more  completely  effaced  in  proportion  to 
the  more  crystalline  texture  of  the  beds,  and  their  vicinity  to  the 
granite.  In  some  places  the  siliceous  matter  of  the  schist  be- 
comes a  granular  quartz,  and  when  hornblende  and  mica  are 
added,  the  altered  rock  loses  its  stratification,  and  passes  into  a 
kind  of  granite.  The  limestone,  which  at  points  remote  from 
the  granite  is  of  an  earthy  texture,  blue  colour,  and  often  abounds 
in  corals,  becomes  a  white  granular  marble  near  the  granite, 
sometimes  siliceous,  the  granular  structure  extending  occasion- 

Fig.  126. 


Altered  zone  offossiliferous  slate  and  limestone  near 


Christiania. 


The  arrows  indicate  the  dip,  and  the  straight  lines  the  strike,  of 
the  beds. 

ally  upwards  of  400  yards  from  the  junction  ;  and  the  corals  be- 
ing for  the  most  part  obliterated,  though  sometimes  preserved, 
even  in  the  white  marble.  Both  the  altered  limestone  and  hard- 
ened slate  contain  garnets  in  many  places,  also  ores  of  iron, 
lead,  and  copper,  with  some  silver.  These  alterations  occur 
equally,  whether  the  granite  invades  the  strata  in  a  line  parallel 
to  the  general  strike  of  the  fossiliferous  beds,  or  in  a  line  at  right 
angles  to  their  strike,  as  will  be  seen  by  the  accompanying  ground 
plan.* 

The  indurated  and  ribboned  schists  above  mentioned,  bear  a 


*  Keilhau,  Gcea  Norvegica,  pp.  61 — 63. 


PART  I.     CHAPTER  XI.  145 

Alterations  of  Strata  in  contact  with  Granite. 

strong  resemblance  to  certain  shales  of  the  coal  found  at  Rus- 
sell's Hall,  near  Dudley,  where  coal  mines  have  been  on  fire  for 
ages.  Beds  of  shale  of  considerable  thickness,  lying  over  the 
burning,  have  been  baked  and  hardened  so  as  to  acquire  a  flinty 
fracture,  the  layers  being  alternately  green  and  brick-coloured. 

The  granite  of  Cornwall,  in  like  manner,  sends  forth  veins 
into  a  coarse  argillaceous-schist,  provincially  termed  killas.  This 
killas  is  converted  into  hornblende-schist  near  the  contact  with 
the  veins.  These  appearances  are  well  seen  at  the  junction  of 
the  granite  and  killas,  in  St.  Michael's  Mount,  a  small  island 
nearly  300  feet  high,  situated  in  the  bay,  at  a  distance  of  about 
three  miles  from  Penzance. 

The  granite  of  Dartmoor,  in  Devonshire,  says  Mr.  De  la 
Beche,  has  intruded  itself  into  the  slate  and  slaty  sandstone 
called  greywacke,  twisting  and  contorting  the  strata,  and  send- 
ing veins  into  them.  Hence  some  of  the  slate  rocks  have  be- 
come "  micaceous,  others  more  indurated,  and  with  the  charac- 
ters of  mica-slate  and  gneiss,  while  others  again  appear  convert- 
ed into  a  hard-zoned  rock  strongly  impregnated  with  felspar."* 

We  learn  from  the  investigations  of  M.  Dufrenoy,  that  in  the 
eastern  Pyrenees  there  are  mountain  masses  of  granite  poste- 
rior in  date  to  the  formation  called  lias  and  chalk  of  that  dis- 
trict, and  that  these  fossiliferous  rocks  are  greatly  altered  in 
texture,  and  often  charged  with  iron-ore,  in  the  neighbourhood 
of  the  granite.  Thus  in  the  environs  of  St.  Martin,  near  St. 
Paul  de  Fenouillet,  the  chalky  limestone  becomes  more  crystal- 
line and  saccharoid  as  it  approaches  the  granite,  and  loses  all 
traces  of  the  fossils  which  it  previously  contained  in  abundance. 
At  some  points  also  it  becomes  dolomitic,  and  filled  with  small 
veins  of  carbonate  of  iron,  and  spots  of  red  iron-ore.  At  Ran- 
cie  the  lias  nearest  the  granite  is  not  only  filled  with  iron-ore, 
but  charged  with  pyrites,  tremolite,  garnet,  and  a  new  mineral 
somewhat  allied  to  felspar,  called,  from  the  place  in  the  Py- 
renees where  it  occurs,  "  couzeranite." 

Now  the  alterations  above  described  as  superinduced  in  rocks 
by  volcanic  dikes  and  granite  veins,  prove  incontestably  that 
powers  exist  in  nature  capable  of  transforming  fossiliferous  into 
crystalline  strata, — powers  capable  of  generating  in  them  a  new 
mineral  character,  similar,  nay,  often  absolutely  identical  with 
that  of  gneiss,  mica-schist,  and  other  stratified  members  of  the 
hypogene  series.  The  precise  nature  of  these  altering  causes, 
which  may  provisionally  be  termed  plutonic,  is  in  a  great  degree 
obscure  and  doubtful ;  but  their  reality  is  no  less  clear,  and  we 

*  Geol.  Manual,  p.  479. 


146  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Rocks  altered  by  Subterranean  Gases. 

must  suppose  the  influence  of  heat  to  be  in  some  way  connected 
with  the  transmutation,  if,  for  reasons  before  explained,  we  con- 
cede the  igneous  origin  of  granite. 

The  experiments  of  Gregory  Watt,  in  fusing  rocks  in  the 
laboratory,  and  allowing  them  to  consolidate  by  slow  cooling, 
prove  distinctly  that  a  rock  need  not  be  perfectly  melted  in  order 
that  a  re-arrangement  of  its  component  particles  should  take 
place,  and  a  partial  crystallization  ensue.*  We  may  easily  sup- 
pose, therefore,  that  all  traces  of  shells  and  other  organic  re- 
mains may  be  destroyed ;  and  that  new  chemical  combinations 
may  arise,  without  the  mass  being  so  fused  as  that  the  lines  of 
stratification  should  be  wholly  obliterated. 

We  must  not,  however,  imagine  that  heat  alone,  such  as  may 
be  applied  to  a  stone  in  the  open  air,  can  constitute  all  that  is 
comprised  in  plutonic  action.  We  know  that  volcanos  in  erup- 
tion not  only  emit  fluid  lava,  but  give  off  steam  and  other  heated 
gases,  which  rush  out  in  enormous  volume,  for  days,  weeks,  or 
years  continuously,  and  are  even  disengaged  from  lava  during 
its  consolidation.  When  the  materials  of  granite,  therefore, 
came  in  contact  with  the  fossiliferous  stratum  in  the  bowels  of 
the  earth  under  great  pressure,  the  contained  gases  might  be  un- 
able to  escape ;  yet  when  brought  into  contact  with  rocks,  might 
pass  through  their  pores  with  greater  facility  than  water  is 
known  to  do.  (See  p.  52.)  These  aeriform  fluids,  such  as  sul- 
phuretted hydrogen,  muriatic  acid,  and  carbonic  acid,  issue  in 
many  places  from  rents  in  rocks,  which  they  have  discoloured 
and  corroded,  softening  some  and  hardening  others.  If  the 
rocks  are  charged  with  water,  they  would  pass  through  more 
readily ;  for,  according  to  the  experiments  of  Henry,  water, 
under  an  hydrostatic  pressure  of  ninety-six  feet,  will  absorb 
three  times  as  much  carbonic  acid  gas  as  it  can  under  the  ordi- 
nary pressure  of  the  atmosphere.  Although  this  increased 
power  of  absorption  would  be  diminished,  in  consequence  of  the 
higher  temperature  found  to  exist  as  we  descend  in  the  earth, 
yet  Professor  Bischoff  has  shown  that  the  heat  by  no  means 
augments  in  such  a  proportion  as  to  counteract  the  effect  of  aug- 
mented pressure."!"  There  are  other  gases,  as  well  as  the  car- 
bonic acid,  which  water  absorbs,  and  more  rapidly  in  proportion 
to  the  amount  of  pressure.  Now  even  the  most  compact  rocks 
may  be  regarded,  before  they  have  been  exposed  to  the  air  and 
dried,  in  the  light  of  sponges  filled  with  water ;  and  it  is  con- 
ceivable that  heated  gases  brought  into  contact  with  them,  at 
great  depths,  may  be  absorbed  readily,  and  transfused  through 

*  Phil.  Trans.  1804.    t  Poggendorf's  Annalen,  No.  XVI.,  Second  Series,  vol.  Hi. 


PART  I.     CHAPTER  XL  147 

Rocks  altered  by  Subterranean  Gases. 

their  pores.  Although  the  gaseous  matter  first  absorbed  would 
soon  be  condensed,  and  part  with  its  heat,  yet  the  continued  arri- 
val of  fresh  supplies  from  below,  might,  in  the  course  of  ages, 
cause  the  temperature  of  the  water,  and  with  it  that  of  the  con- 
taining rock,  to  be  materially  raised. 

M.  Fournet,  in  his  description  of  the  metalliferous  gneiss  near 
Clermont,  in  Auvergne,  states  that  all  the  minute  fissures  of  the 
rock  are  quite  saturated  with  free  carbonic  acid  gas,  which  rises 
plentifully  from  the  soil  there  and  in  many  parts  of  the  surround- 
ing country.  The  various  elements  of  the  gneiss,  with  the  ex- 
ception of  the  quartz,  are  all  softened ;  and  new  combinations 
of  the  acid,  with  lime,  iron,  arid  manganese,  are  continually  in 
progress.* 

Another  illustration  of  the  power  of  subterranean  gases  is  af- 
forded by  the  stufas  of  St.  Calogero,  situated  in  the  largest  of  the 
Lipari  Islands.  Here,  according  to  the  description  lately  pub- 
lished by  Hoffmann,  horizontal  strata  of  tuff,  extending  for  four 
miles  along  the  coast,  and  forming  cliffs  more  than  200  feet  high, 
have  been  discoloured  in  various  places,  and  strangely  altered 
by  the  "  all-penetrating  vapours."  Dark  clays  have  become  yel- 
low, or  often  snow-white ;  or  have  assumed  a  chequered  and 
brecciated  appearance,  being  crossed  with  ferruginous  red  stripes. 
In  some  places  the  fumeroles  have  been  found  by  analysis  to  con- 
sist partly  of  sublimations  of  oxide  of  iron  ;  but  it  also  appears 
that  veins  of  calcedony  and  opal,  and  others  of  fibrous  gypsum, 
have  resulted  from  these  volcanic  exhalations.f 

The  reader  may  also  refer  to  M.  Virlet's  account  of  the  corro- 
sion of  hard,  flinty,  and  jaspideous  rocks  near  Corinth,  by  the 
prolonged  agency  of  subterranean  gases  ;  J  and  to  Dr.  Daubeny's 
description  of  the  decomposition  of  trachyte  rocks  in  the  Solfa- 
tara,  near  Naples,  by  sulphuretted  hydrogen  and  muriatic  acid 

*•§ 

Although  in  all  these  instances  we  can  only  study  the  phe- 
nomena as  exhibited  at  the  surface,  it  is  clear  that  the  gaseous 
fluids  must  have  made  their  way  through  the  whole  thickness  of 
porous  or  fissured  rocks,  which  intervene  between  the  subterra- 
nean reservoirs  of  gas  and  the  external  air.  The  extent,  there- 
fore, of  the  earth's  crust,  which  the  vapours  have  permeated  and 
are  now  permeating,  may  be  thousands  of  fathoms  in  thickness, 

*  See  Principles  of  Geology,  Index,  "  Auvergne,"  &c. 
t  Hoffmann's  Liparischen  Inseln,  p.  38.  Leipzig,  1832. 

I  See  Principles  of  Geolgy ;  and  Bulletin  de  la  Soc.  Geol.  de  France,  torn.  ii. 
p.  330. 
$  See  Principles  of  Geology ;  and  Daubeny's  Volcanos,  p.  167. 


148       LYELL'S  ELEMENTS  OF  GEOLOGY. 


Origin  of  Metamorphic  Structure. 


and  their  heating  and  modifying  influence  may  be  spread  through- 
out the  whole  of  the  solid  mass. 

The  above  observations  are  calculated  to  meet  some  of  the 
objections  which  have  been  urged  against  the  metamorphic  the- 
ory on  the  ground  of  the  small  power  of  rocks  to  conduct  heat; 
for  it  is  well  known  that  rocks,  when  dry  and  in  the  air,  differ 
remarkably  from  metals  in  this  respect.  It  has  been  asked  how 
the  changes  which  extend  merely  for  a  few  feet  from  the  con- 
tact of  a  dike  could  have  penetrated  through  mountain  masses 
of  crystalline  strata  several  miles  in  thickness.  Now  it  has  been 
stated  that  the  plutonic  influence  of  the  syenite  of  Norway,  has 
sometimes  altered  fossiliferous  strata  for  a  distance  of  a  quarter 
of  a  mile,  both  in  the  direction  of  their  dip  and  of  their  strike. 
(See  Fig.  126.  p.  144.)  This  is  undoubtedly  an  extreme  case; 
but  is  it  not  far  more  philosophical  to  suppose  that  this  influence 
may,  under  favourable  circumstances,  affect  denser  masses,  than 
to  invent  an  entirely  new  cause  to  account  for  effects  merely  dif- 
fering in  quantity,  and  not  in  kind  ?  The  metamorphic  theory 
does  not  require  us  to  affirm  that  some  contiguous  mass  of  gra- 
nite has  been  the  altering  power ;  but  merely  that  an  action,  ex- 
isting in  the  interior  of  the  earth  at  an  unknown  depth,  whether 
thermal,  electrical,  or  other,  analogous  to  that  exerted  near  in- 
truding masses  of  granite,  has,  in  the  course  of  vast  and  indefi- 
nite periods,  and  when  rising  perhaps  from  a  large  heated  sur- 
face, reduced  strata  thousands  of  yards  thick  to  a  state  of  semi- 
fusion,  so  that  on  cooling  they  have  become  crystalline,  like 
gneiss.  Granite  may  have  been  another  result  of  the  same  ac- 
tion in  a  higher  state  of  intensity,  by  which  a  thorough  fusion 
has  been  produced  ;  and  in  this  manner  the  passage  from  granite 
into  gneiss  may  be  explained. 

Some  geologists  are  of  opinion,  that  the  alternate  layers  of 
mica  and  quartz,  or  mica  and  felspar,  or  lime  and  felspar,  are 
so  much  more  distinct  in  certain  metamorphic  rocks,  than  the 
ingredients  composing  alternate  layers  in  many  sedimentary  de- 
posits, that  the  similar  particles  must  be  supposed  to  have  exert- 
ed a  molecular  attraction  for  each  other,  and  to  have  thus  con- 
gregated together  in  layers,  more  distinct  in  mineral  composition 
than  before  they  were  crystallized. 

In  considering,  then,  the  various  data  already  enumerated,  the 
forms  of  stratification  in  metamorphic  rocks,  their  passage  on  the 
one  hand  into  the  fossiliferous.  and  on  the  other  into  the  plutonic 
formations,  and  the  conversions  which  can  be  ascertained  to  have 
occurred  in  the  vicinity  of  granite,  we  may  conclude  that  gneiss 
and  mica-schist  may  be  nothing  more  than  altered  micaceous 
and  argillaceous  sandstones,  that  granular  quartz  may  have  been 


PART  I.     CHAPTER  XL  149 

Origin  of  Metamorphic  Rocks. 

derived  from  siliceous  sandstone,  and  compact  quartz  from  the 
same  materials.  Clay-slate  may  be  altered  shale,  and  granular 
marble  may  have  originated  in  the  form  of  ordinary  limestone, 
replete  with  shells  and  corals,  which  have  since  been  obliterated; 
and,  lastly,  calcareous  sands  and  marls  may  have  been  changed 
into  impure  crystalline  limestones. 

"  Hornblende-schist,"  says  Dr.  MacCulloch,  "  may  at  first 
have  been  mere  clay ;  for  clay  or  shale  is  found  altered  by  trap 
into  Lydian  stone,  a  substance  differing  from  hornblende-schist 
almost  solely  in  compactness  and  uniformity  of  texture."*  "  In 
Shetland,"  remarks  the  same  author,  "argillaceous-schist  (or 
clay-slate,)  when  in  contact  with  granite,  is  sometimes  converted 
into  hornblende-schist,  the  schist  becoming  first  siliceous,  and 
ultimately,  at  the  contact,  hornblende-schist."f 

The  anthracite  found  associated  with  hypogene  rocks  may 
have  been  coal ;  for  we  know  that,  in  the  vicinity  of  some  trap 
dikes,  coal  is  converted  into  anthracite. 

The  total  absence  of  any  trace  of  fossils  has  inclined  many 
geologists  to  attribute  the  origin  of  crystalline  strata  to  a  period 
antecedent  to  the  existence  of  organic  beings.  Admitting,  they 
say,  the  obliteration,  in  some  cases,  of  fossils  by  plutonic  action, 
we  might  still  expect  that  traces  of  them  would  oftener  occur  in 
certain  ancient  systems  of  slate,  in  which,  as  in  Cumberland, 
some  conglomerates  occur.  But  in  urging  this  argument,  it  seems 
to  have  been  forgotten,  that  there  are  stratified  formations  of 
enormous  thickness,  and  of  various  ages,  and  some  of  them  very 
modern,  all  formed  after  the  earth  had  become  the  abode  of  liv- 
ing creatures,  which  are  nevertheless  in  certain  districts  entirely 
destitute  of  all  vestiges  of  organic  bodies.  In  some,  the  traces 
of  fossils  may  have  been  effaced  by  water  and  acids,  at  many 
successive  periods ;  and  it  is  clear,  that  the  older  the  stratum, 
the  greater  is  the  chance  of  its  being  non-fossiliferous,  even  if  it 
has  escaped  all  metamorphic  action. 

It  has  been  also  objected  to  the  metamorphic  theory,  that  the 
chemical  composition  of  the  secondary  strata  differs  essentially 
from  that  of  the  crystalline  schists,  into  which  they  are  supposed 
to  be  convertible.^:  The  "primary"  schists,  it  is  said,  usually 
contain  a  considerable  proportion  of  potash  or  of  soda,  which  the 
secondary  clays,  shales,  and  slates  do  not,  these  last  being  the 
result  of  the  decomposition  of  felspathic  rocks,  from  which  the 
alkaline  matter  has  been  abstracted  during  the  process  of  decom= 
position.  But  this  reasoning  proceeds  on  insufficient  and  appa- 

*  Syst.  of  GeoL,  vol.  i.  p.  210.  t  Ibid.,  p.  211. 

t  Dr.  Boase,  Primary  Geology,  p.  319. 

N* 


150      LYELL'S  ELEMENTS  OF  GEOLOGY. 


Objections  to  the  Metamorphic  Theory. 


rently  mistaken  data ;  for  a  large  portion  of  what  is  usually  called 
clay,  marl,  shale,  and  slate  does  actually  contain  a  certain  and 
often  a  considerable  proportion  of  alkali ;  so  that  it  is  difficult  in 
many  countries  to  obtain  clay  or  shale  sufficiently  free  from 
alkaline  ingredients  to  allow  of  their  being  burnt  into  bricks  or 
used  for  pottery. 

Thus  the  argillaceous  shales,  as  they  are  called,  and  slates 
of  the  old  red  sandstone,  in  Forfarshire  and  other  parts  of  Scot- 
land, are  so  much  charged  with  alkali,  derived  from  triturated 
felspar,  that,  instead  of  hardening  when  exposed  to  fire,  they 
melt  readily  into  a  glass.  They  contain  no  lime,  but  appear  to 
consist  of  extremely  minute  grains  of  the  various  ingredients  of 
granite,  which  are  distinctly  visible  in  the  coarser-grained  vari- 
eties, and  in  almost  all  the  interposed  sandstones.  These  lami- 
nated clays,  marls,  and  shales  might  certainly,  if  crystallized, 
resemble  in  composition  many  of  the  primary  strata. 

There  is  also  potash  in  the  vegetable  remains  included  in  strata, 
and  soda  in  the  salts  by  which  they  are  sometimes  so  largely 
impregnated,  as  in  Patagonia. 

Another  objection  has  been  derived  from  the  alternation  of 
highly  crystalline  strata  with  others  having  a  less  crystalline  tex- 
ture. The  heat,  it  is  said,  in  its  ascent  from  below,  must  have 
traversed  the  less  altered  schists  before  it  reached  a  higher  and 
more  crystalline  bed.  In  answer  to  this,  it  may  be  observed, 
that  if  a  number  of  strata  differing  greatly  in  composition  from 
each  other  be  subjected  to  equal  quantities  of  heat,  there  is  every 
probability  that  some  will  be  more  fusible  than  others.  Some, 
for  example,  will  contain  soda,  potash,  lime,  or  some  other  ingre- 
dient capable  of  acting  as  a  flux  ;  while  others  may  be  destitute 
of  the  same  elements,  and  so  refractory  as  to  be  very  slightly 
affected  by  a  degree  of  heat  capable  of  reducing  others  to  semi- 
fusion.  Nor  should  it  be  forgotten  that,  as  a  general  rule,  the 
less  crystalline  rocks  do  really  occur  in  the  upper,  and  the  more 
crystalline  in  the  lower  part  of  each  metamorphic  series. 

But  it  will  be  impossible  for  the  reader  duly  to  appreciate  the 
propriety  of  the  term  metamorphic,  as  applied  to  the  strata  hith- 
erto called  primary,  until  I  have  shown  in  the  second  part  of  this 
work,  that  these  crystalline  strata  have  been  formed  at  a  great 
variety  of  distinct  periods. 


ELEMENTS  OF  GEOLOGY. 


PART  II. 


CHAPTER  XII. 


ON   THE   DIFFERENT  AGES    OF   THE   FOUR   GREAT  CLASSES   OF   ROCKS. 

Aqueous,  plutonic,  volcanic,  and  metamorphic  rocks,  considered  chronologi- 
cally— Lehman's  division  into  primitive  and  secondary — Werner's  addition  of  a 
transition  class — Neptunian  theory — Hutton  on  igneous  origin  of  granite — How 
the  name  of  primary  was  still  retained  for  granite — The  term  "transition,"  why 
faulty — The  adherence  to  the  old  chronological  nomenclature  retarded  the  pro- 
gress of  geology — New  hypothesis  invented  to  reconcile  the  igneous  origin  of 
granite  to  the  notion  of  its  high  antiquity — Explanation  of  the  chronological  no- 
menclature adopted  in  this  work,  so  far  as  regards  primary,  secondary,  and  ter- 
tiary periods. 

IN  the  first  part  of  this  work  the  four  great  classes  of  rocks, 
the  aqueous,  the  volcanic,  the  plutonic  and  the  metamorphic, 
have  been  considered  with  reference  to  their  external  characters, 
their  mineral  composition,  and  mode  of  origin ;  and  it  now  re- 
mains to  treat  of  the  same  classes  with  reference  to  the  different 
periods  at  which  they  were  formed.  In  speaking  of  the  aqueous 
rocks,  for  example,  it  has  been  shown  that  they  are  stratified, 
that  some  are  calcareous,  others  argillaceous,  some  made  up  of 
sand,  others  of  pebbles ;  that  some  contain  freshwater,  others 
marine  fossils,  and  so  forth ;  but  the  student  has  still  to  learn 
which  rocks,  exhibiting  some  or  all  of  these  characters,  have  ori- 
ginated at  one  "period  of  the  earth's  history,  and  which  at  an- 
other. 

So  in  regard  to  the  volcanic  and  plutonic  formations,  we  have 
hitherto  examined  their  mineral  peculiarities,  forms,  and  mode 
of  origin,  but  have  still  to  inquire  into  their  chronological  his- 
tory. 

Lastly,  a  more  curious  question  will  demand  our  attention, 
when  we  endeavour  to  ascertain  the  relative  ages  of  the  meta- 
morphic rocks,  the  chronology  of  which  may  be  said  to  be  two- 
fold, each  formation  having  been  deposited  at  one  period,  and 
having  assumed  a  crystalline  texture  at  another. 

*  (151) 


152      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Age  of  Rocks Lehman's  and  Werner's  Classification  of  Rocks. 

It  was  for  many  years  a  received  opinion,  that  the  formation 
of  whole  classes  of  rocks,  such  as  the  plutonic  and  metamorphic, 
began  and  ended  before  any  members  of  the  aqueous  and  volca- 
nic orders  were  produced ;  and  although  this  idea  has  long  been 
modified,  and  is  nearly  exploded,  it  will  be  necessary  to  give 
some  account  of  the  ancient  doctrine,  in  order  that  beginners 
may  understand  whence  part  of  the  nomenclature  of  geology  still 
partially  in  use  was  derived. 

About  the  middle  of  the  last  century,  Lehman,  a  German  mi- 
ner, proposed  to  divide  rocks  into  three  classes,  the  first  and  old- 
est to  be  called  primitive,  comprising  the  plutonic  and  metamor- 
phic rocks ;  the  next  to  be  termed  secondary,  comprehending  the 
aqueous  or  fossiliferous  strata ;  and  the  remainder  or  third  class, 
the  supposed  effect  of  "  local  floods,  and  the  deluge  of  Noah," 
corresponding  to  our  alluvium,  ancient  and  mordern.  In  the 
primitive  class,  he  said,  such  as  granite  and  gneiss,  there  are  no 
organic  remains,  nor  any  signs  of  materials  derived  from  the 
ruins  of  pre-existing  rocks.  Their  origin,  therefore,  may  have 
been  purely  chemical,  antecedent  to  the  creation  of  living  beings, 
and  probably  coeval  with  the  birth  of  the  world  itself.  The  se- 
condary formations,  on  the  contrary,  which  often  contain  sand, 
pebbles,  and  organic  remains,  must  have  been  mechanical  de- 
posits, produced  after  the  planet  had  become  the  habitation  of 
animals  and  plants-  This  bold  generalization,  although  antici- 
pated in  some  measure  by  Steno,  a  century  before,  in  Italy, 
formed  at  the  time  an  importanPstep  in  the  progress  of  geology, 
and  sketched  out  correctly  some  of  the  leading  divisions  into 
which  rocks  may  be  separated.  About  half  a  century  later, 
Werner,  so  justly  celebrated  for  his  improved  methods  of  dis- 
criminating the  mineralogical  characters  of  rocks,  attempted  to 
improve  Lehman's  classification,  and  with  this  view  intercalated 
a  class,  called  by  him  "  the  transition  formations,"  between  the 
primitive  and  secondary.  Between  these  last  he  had  discovered, 
in  northern  Germany,  a  series  of  strata,  which  in  their  mineral 
peculiarities  were  of  an  intermediate  character,  partaking  in  some 
degree  of  the  crystalline  nature  of  micaceous  and  clay-slate,  and 
yet  exhibiting  here  and  there  signs  of  a  mechanical  origin  and 
organic  remains.  For  this  group,  therefore,  forming  a  passage 
between  Lehman's  primitive  and  secondary  rocks,  the  name  of 
transition  was  proposed.  They  consisted  principally  of  clay- 
slate  and  an  argillaceous  sandstone,  called  greywacke  and  partly 
of  calcareous  beds.  It  happened  in  the  district  which  Werner 
first  investigated,  that  both  the  primitive  and  transition  strata 
were  highly  inclined,  while  the  beds  of  the  newer  and  fossilife- 
rous rocks  were  horizontal.  To  these  latter,  therefore,  he  gave 


PART  II.     CHAPTER  XII.  153 


Neptunian  Theory Button  on  Granite. 


the  name  of/?ote,  or  flat ;  and  every  deposit  more  modern  than 
the  chalk,  or  uppermost  of  the  flotz  series,  was  designated  "  the 
overflowed  land,"  an  expression  which  may  be  regarded  as 
equivalent  to  alluvium.  As  the  followers  of  Werner  soon  dis- 
covered that  the  inclined  position  of  the  "  transition  beds,"  and 
the  horizontality  of  the  flotz,  or  newer  fossiliferous  strata,  were 
mere  local  accidents,  they  soon  abandoned  the  term  flotz ;  and 
the  four  divisions  of  the  Wernerian  school  were  then  named  pri- 
mitive, transition,  secondary,  and  alluvium. 

As  to  the  trappean  rocks,  although  their  igneous  origin  had 
been  already  demonstrated  by  Arduino,  Fortis,  Faujas,  and 
others,  and  especially  by  Desmarest,  they  were  all  regarded  by 
Werner  as  aqueous,  and  as  mere  subordinate  members  of  the 
secondary  formations.* 

This  theory  of  Werner's  was  called  the  "  Neptunian,"  and  for 
many  years  enjoyed  much  popularity.  It  assumed  that  the 
globe  had  been  at  first  invested  by  an  universal  chaotic  ocean, 
holding  the  materials  of  all  rocks  in  solution.  From  the  waters 
of  this  ocean,  granite,  gneiss,  and  other  crystalline  formations, 
were  first  precipitated ;  and  afterwards,  when  the  waters  were 
purged  of  these  ingredients,  and  more  nearly  resembled  those  of 
our  actual  seas,  the  transition  strata  were  deposited.  These  were 
of  a  mixed  character,  not  purely  chemical,  because  the  waves 
and  currents  had  already  begun  to  wear  down  solid  land,  and 
to  give  rise  to  pebbles,  sand,  and  mud ;  nor  entirely  without  fos- 
sils, because  a  few  of  the  first  marine  animals  had  begun  to  ex- 
ist. After  this  period,  the  secondary  formations  were  accumu- 
lated in  waters  resembling  those  of  the  present  ocean,  except  at 
certain  intervals,  when,  from  causes  wholly  unexplained,  a  par- 
tial recurrence  of  the  "chaotic  fluid"  took  place,  during  which 
various  trap  rocks,  some  highly  crystalline,  were  formed.  This 
arbitrary  hypothesis  rejected  all  intervention  of  igneous  agency, 
volcanos  being  regarded  as  partial  and  superficial  accidents,  of 
trifling  account  among  the  great  causes  which  have  modified  the 
external  structure  of  the  globe. 

Meanwhile  Hutton,  a  contemporary  of  Werner,  began  to  teach, 
in  Scotland,  that  granite  as  well  as  trap  was  of  igneous  origin, 
and  had  at  various  periods  intruded  itself  in  a  fluid  state  into  dif- 
ferent parts  of  the  earth's  crust.  He  recognized  and  faithfully 
described  many  of  the  phenomena  of  granitic  veins,  and  the  al- 
terations produced  by  them  on  the  invaded  strata,  which  have 
been  treated  of  in  the  ninth  chapter.  He,  moreover,  advanced 
the  opinion,  that  the  crystalline  strata  called  primitive  had  not 

*  See  Principles  of  Geology,  vol.  i.  chap.  iv. 


154      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Term  Transition,  why  objectionable. 

been  precipitated  from  a  primeval  ocean,  but  were  sedimentary 
strata  altered  by  heat.  In  his  writings,  therefore,  and  in  those 
of  his  illustrator,  Playfair,  we  find  the  germ  of  that  metamorphic 
theory  which  has  been  already  expounded.* 

At  length,  after  much  controversy,  the  doctrine  of  the  igneous 
origin  of  trap  and  granite  made  its  way  into  general  favour ; 
but  although  it  was,  in  consequence,  admitted  that  both  granite 
and  trap  had  been  produced  at  many  successive  periods,  the  term 
primitive  or  primary  still  continued  to  be  applied  to  the  crystal- 
line formations  in  general,  whether  stratified,  like  gneiss,  or  un- 
stratified,  like  granite.  The  pupil  was  told  that  granite  was  a 
primary  rock,  but  that  some  granites  were  newer  than  certain 
secondary  formations ;  and  in  conformity  with  the  spirit  of  the 
ancient  language,  to  which  the  teacher  was  still  determined  to 
adhere,  a  desire  was  naturally  engendered  of  extenuating  the 
importance  of  those  more  modern  granites  which  new  observa- 
tions were  continually  bringing  to  light. 

A  no  less  decided  inclination  was  shown  to  persist  in  the  use 
of  the  term  "transition,"  after  it  had  been  proved  to  be  almost 
as  faulty  in  its  original  application  as  that  of  flotz.  The  name 
of  transition,  as  already  stated,  was  first  given  by  Werner,  to 
designate  a  mineral  character,  intermediate  between  the  meta- 
morphic state  and  that  of  an  ordinary  fossiliferous  rock.  But 
the  term  acquired  also  from  the  first  a  chronological  import,  be- 
cause it  had  been  appropriated  to  sedimentary  formations,  which, 
in  the  Hartz  and  other  parts  of  Germany,  were  more  ancient 
than  the  oldest  of  the  secondary  series,  and  were  characterized 
by  peculiar  fossil  zoophytes  and  shells.  When,  therefore,  geol- 
ogists found  in  other  districts  stratified  rocks  occupying  the  same 
position,  and  inclosing  similar  fossils,  they  gave  to  them  also  the 
name  of  transition,  according  to  rules  which  will  be  explained 
in  the  next  chapter ;  yet,  in  many  cases,  such  rocks  were  found 
not  to  exhibit  the  same  mineral  texture  which  Werner  had  called 
transition.  On  the  contrary,  many  of  them  were  not  more  crys- 
talline than  dhTerent  members  of  the  secondary  class ;  while,  on 
the  other  hand,  these  last  were  sometimes  found  to  assume  a 
semi-crystalline  and  almost  metamorphic  aspect,  and  thus,  on 
lithological  grounds,  to  deserve  equally  the  name  of  transition. 
So  remarkably  was  this  the  case  in  the  Swiss  Alps,  that  certain 
rocks,  which  had  for  years  been  regarded  by  some  of  the  most 
skilful  disciples  of  Werner  to  be  transition,  were  at  last  acknow- 
ledged, when  their  relative  position  and  fossils  were  better  un- 
derstood, to  belong  to  the  newest  of  the  secondary  groups !  If 

»  j. 

*  See  chapters  X.  and  XL 


PART  II.     CHAPTER  XII.  155 


Neptunian  Theory  of  the  Origin  of  Granite. 


under  such  circumstances  the  name  of  transition  was  retained,  it 
is  clear  that  it  ought  to  have  been  applied  without  reference  to 
the  age  of  strata,  and  simply  as  expressive  of  a  mineral  pecu- 
liarity. The  continued  appropriation  of  the  term  to  formations 
of  a  given  date,  induced  geologists  to  go  on  believing  that  the 
ancient  strata  so  designated  bore  a  less  resemblance  to  the  se- 
condary than  is  really  the  case,  and  to  imagine  that  these  last 
never  pass,  as  they  frequently  do,  into  metamorphic  rocks. 

The  poet  Waller,  when  lamenting  over  the  antiquated  style  of 
Chaucer,  complains  that  — 

We  write  in  sand,  our  language  grows, 
And,  like  the  tide,  our  work  o'erflows ; 

But  the  reverse  is  true  in  geology ;  for  here  it  is  our  work 
which  continually  outgrows  the  language.  The  tide  of  observation 
advances  with  such  speed,  that  improvements  in  theory  outrun 
the  changes  of  nomenclature  ;  and  the  attempt  to  inculcate  new 
truths  by  words  invented  to  express  a  different  or  opposite  opin- 
ion, tends  constantly,  by  the  force  of  association,  to  perpetuate 
error ;  so  that  dogmas  renounced  by  the  reason  still  retain  a 
strong  hold  upon  the  imagination. 

In  order  to  reconcile  the  old  chronological  views  with  the  new 
doctrine  of  the  igneous  origin  of  granite,  the  following  hypothe- 
sis was  substituted  for  that  of  the  Neptunists.  Instead  of  begin- 
ning with  an  aqueous  menstruum  or  chaotic  fluid,  the  materials 
of  the  present  crust  of  the  earth  were  supposed  to  have  been  at 
first  in  a  state  of  igneous  fusion,  until  part  of  the  heat  having 
been  diffused  into  surrounding  space,  the  surface  of  the  fluid  con- 
solidated, and  formed  a  crust  of  granite.  This  covering  of  crys- 
talline stone,  which  afterwards  grew  thicker  and  thicker  as  it 
cooled,  was  so  hot,  at  first,  that  no  water  could  exist  upon  it ;  but 
as  the  refrigeration  proceeded,  the  aqueous  vapour  in  the  atmo- 
sphere was  condensed,  and,  falling  in  rain,  gave  rise  to  the  first 
thermal  ocean.  So  high  was  the  temperature  of  this  boiling  sea, 
that  no  aquatic  beings  could  inhabit  its  waters,  and  its  deposits 
were  not  only  devoid  of  fossils,  but,  like  those  of  some  hot  springs, 
were  highly  crystalline.  Hence  the  origin  of  the  primary  or 
crystalline  strata. 

Afterwards,  when  the  granitic  crust  had  been  partially  broken 
up,  land  and  mountains  began  to  rise  above  the  waters,  and  rains 
and  torrents  ground  down  rock,  so  that  sediment  was  spread  over 
the  bottom  of  the  seas.  Yet  the  heat  still  remaining  in  the  solid 
supporting  substances  was  sufficient  "to  increase  the  chemical 
action  exerted  by  the  water,  although  not  so  intense  as  to  pre- 
vent the  introduction  and  increase  of  some  living  beings.  During 


156  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Simultaneous  Origin  of  the  four  Classes  of  Rocks. 

this  state  of  things  some  of  the  residuary  mineral  ingredients  of 
the  primeval  ocean  were  precipitated,  and  formed  deposits  (the 
transition  strata  of  Werner),  half  chemical  and  half  mechanical, 
and  containing  a  few  fossils. 

By  this  new  theory,  which  was  in  part  a  revival  of  the  doc- 
trine of  Leibnitz,  published  in  1680,  on  the  igneous  origin  of  the 
planet,  the  old  ideas  respecting  the  priority  of  all  crystalline  rocks 
to  the  creation  of  organic  beings,  were  still  preserved ;  and  the 
notion,  that  all  the  semi-crystalline  and  partially  fossiliferous 
rocks  belonged  to  one  period,  while  all  the  earthy  and  uncrys- 
talline  formations  originated  at  a  subsequent  epoch,  was  also 
perpetuated. 

It  may  or  may  not  be  true,  as  the  great  Leibnitz  imagined, 
that  the  whole  planet  was  once  in  a  state  of  liquefaction  by  heat ; 
but  there  are  certainly  no  geological  proofs  that  the  granite  which 
constitutes  the  foundation  of  so  much  of  the  earth's  crust  was 
ever  in  a  state  of  universal  fusion.  On  the  contrary,  all  our 
evidence  tends  to  show  that  the  formation  of  granite,  like  the 
deposition  of  the  stratified  rocks,  has  been  successive,  and  that 
different  portions  of  granite  have  been  in  a  melted  state  at  dis- 
tinct and  often  distant  periods.  One  mass  was  solid,  and  had 
been  fractured,  before  another  body  of  granitic  matter  was  in- 
jected into  it,  or  through  it,  in  the  form  of  veins.  In  short,  the 
universal  fluidity  of  the  crystalline  foundations  of  the  earth's 
crust,  can  only  be  understood  in  the  same  sense  as  the  univer- 
sality of  the  ancient  ocean.  All  the  land  has  been  under  water, 
but  not  all  at  one  time ;  so  all  the  subterranean  unstratified  rocks 
to  which  man  can  obtain  access  have  been  melted,  but  not  simul- 
taneously. 

In  the  present  work  the  four  great  classes  of  rocks,  the  aqueous, 
plutonic,  volcanic,  and  metamorphic,  will  form  four  parallel,  or 
nearly  parallel,  columns  in  one  chronological  table.  They  will  be 
considered  as  four  sets  of  monuments  relating  to  four  contem- 
poraneous, or  nearly  contemporaneous,  series  of  events.  I  have 
endeavoured,  in  the  Frontispiece,  to  express  the  manner  in  which 
members  of  each  of  the  four  classes  may  have  originated  simul- 
taneously at  every  geological  period.  According  to  this  view,  the 
earth's  crust  may  have  been  continually  remodelled,  above  and 
below,  by  aqueous  and  igneous  causes,  from  times  indefinitely  re- 
mote. In  the  same  manner  as  aqueous  and  fossiliferous  strata 
are  now  formed  in  certain  seas  or  lakes,  while  in  other  places 
volcanic  rocks  break  out  a,t  the  surface,  and  are  connected  with 
reservoirs  of  melted  matter  at  vast  depths  in  the  bowels  of  the 
earth, — so,  at  every  era  of  the  past,  fossiliferous  deposits  and 
superficial  igneous  rocks  were  in  progress  contemporaneously 


PART  II.     CHAPTER  XII.  157 


,      Chronological  Arrangement  of  the  four  Classes  of  Rocks. 

with  others  of  subterranean  and  plutonic  origin,  and  some  sedi- 
mentary strata  were  exposed  to  heat  and  made  to  assume  a 
crystalline  or  metamorphic  structure. 

It  can  by  no  means  be  taken  for  granted,  that  during  all  these 
changes  the  solid  crust  of  the  earth  has  been  increasing  in 
thickness.  It  has  been  shown,  that  so  far  as  aqueous  action  is 
concerned,  the  gain  by  fresh  deposits,  and  the  loss  by  denuda- 
tion, must  at  each  period  have  been  equal ;  and  in  like  manner, 
in  the  inferior  portion  of  the  earth's  crust,  the  acquisition  of  new 
crystalline  rocks,  at  each  successive  era,  may  merely  have  coun- 
terbalanced the  loss  sustained  by  the  melting  of  materials  pre- 
viously consolidated.  As  to  the  relative  antiquity  of  the  crys- 
talline foundations  of  the  earth's  crust,  when  compared  to  the 
fossiliferous  and  volcanic  rocks  which  they  support,  I  have  al- 
ready stated,  in  the  first  chapter,  that  to  pronounce  an  opinion 
on  this  matter  is  as  difficult  as  at  once  to  decide  which  of  the 
two,  whether  the  foundations  or  superstructure  of  an  ancient  city 
built  on  wooden  piles  may  be  the  oldest.  We  have  seen  that  to 
answer  this  question,  we  must  first  be  prepared  to  say  whether 
the  work  of  decay  and  restoration  had  gone  on  most  rapidly 
above  or  below,  whether  the  average  duration  of  the  piles  has 
exceeded  that  of  the  stone  buildings,  or  the  contrary.  So  also 
in  regard  to  the  relative  age  of  the  superior  and  inferior  portions 
of  the  earth's  crust ;  we  cannot  hazard  even  a  conjecture  on  this 
point,  until  we  know  whether,  upon  an  average,  the  power  of 
water  above,  or  that  of  fire  below,  is  most  efficacious  in  giving 
new  forms  to  solid  matter. 

After  the  observations  which  have  now  been  made,  the  reader 
will  perceive  that  the  term  primary  must  either  be  entirely  re- 
nounced, or,  if  retained,  must  be  differently  defined,  and  not 
made  to  designate  a  set  of  crystalline  rocks,  some  of  which 
may  be  newer  than  the  secondary  formations.  In  this  work  I 
shall  follow  most  nearly  the  method  proposed  by  Mr.  Boue,  who 
has  called  all  fossiliferous  rocks  older  than  the  secondary  by  the 
name  of  primary,  which  thus  becomes  a  substitute  for  the  term 
transition,  so  far  as  regards  the  aqueous  strata.  To  prevent 
confusion,  however,  I  shall  always  speak  of  these  as  the  primary 
fossiliferous  formations,  because  the  word  primary  has  hitherto 
been  almost  inseparably  connected  with  the  idea  of  a  non-fossili- 
ferous  rock. 

If  we  can  prove  any  plutonic,  volcanic,  or  metamorphic  rocks 
to  be  older  than  the  secondary  formations,  such  rocks  will  also 
be  primary,  according  to  this  system.  Mr.  Boue  having  with  great 
propriety  excluded  the  metamorphic  rocks,  as  a  class,  from  the 
primary  formations,  proposed  to  call  them  all  "crystalline 


158      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Ages  of  Rocks. 

schists,"  restricting  the  name  of  primary  to  the  older  fossilifer- 
ous  or  transition  strata. 

As  there  are  secondary  fossiliferous  strata,  so  we  shall  find 
that  there  are  plutonic,  volcanic,  and  metamorphic  rocks  of  con- 
temporaneous origin,  which  I  shall  also  term  secondary. 

In  the  next  chapter  it  will  be  shown  that  the  strata  above  the 
chalk  have  been  called  tertiary.  If,  therefore,  we  discover  any 
volcanic,  plutonic,  or  metamorphic  rocks,  which  have  originated 
since  the  deposition  of  the  chalk,  these  also  will  rank  as  tertiary 
formations. 

It  may  perhaps  be  suggested  that  some  metamorphic  strata, 
and  some  granites,  may  be  anterior  in  date  to  the  oldest  of  the 
primary  fossiliferous  rocks.  The  opinion  is  certainly  not  impro- 
bable, and  will  be  discussed  in  future  chapters ;  but  I  may  here 
observe,  that  when  we  arrange  the  four  classes  of  rocks  in  four 
parallel  columns  in  one  table  of  chronology,  it  is  by  no  means 
assumed  that  these  columns  are  all  of  equal  length ;  one  may 
begin  at  an  earlier  period  than  the  rest,  and  another  may  come 
down  to  a  later  point  of  time.  In  the  small  part  of  the  globe 
hitherto  examined,  it  is  hardly  to  be  expected  that  we  should 
have  discovered  either  the  oldest  or  the  newest  of  all  the  four 
classes  of  rocks.  Thus,  if  there  be  primary,  secondary,  and 
tertiary  rocks  of  the  fossiliferous  class,  and  in  like  manner  pri- 
mary, secondary,  and  tertiary  plutonic  formations,  we  may  not 
be  yet  acquainted  with  the  most  ancient  of  the  primary  fossili- 
ferous beds,  or  with  the  newest  of  the  plutonic,  and  so  of  the 
rest. 


PART  II.     CHAPTER  XIII.  159 


Tests  of  the  different  Agea  of  Aqueous  Rocks. 


CHAPTER  XIII. 


ON  THE  DIFFERENT  AGES  OF  THE  AQUEOUS  ROCKS. 

On  the  three  principal  tests  of  relative  age — superposition,  mineral  character, 
and  fossils  —  Change  of  mineral  character  and  fossils  in  the  same  continuous 
formation  —  Proofs  that  distinct  species  of  animals  and  plants  have  lived  at  suc- 
cessive periods — Test  of  age  by  included  fragments — Frequent  absence  of  strata 
of  intervening  periods — Principal  groups  of  strata  in  western  Europe — Tertiary 
strata  separable  into  four  groups,  the  fossil  shells  of  which  approach  nearer  to 
those  now  living  in  proportion  as  the  formation  is  more  modern — Terms  Eocene, 
Miocene,  and  Pliocene  —  Identifications  of  fossil  and  recent  shells  by  M.  De- 
shayes — Opinions  of  Dr.  Beck. 

IN  the  last  chapter  I  spoke  generally  of  the  chronological  re- 
lations of  the  four  great  classes  of  rocks,  and  I  shall  now  treat 
of  the  aqueous  rocks  in  particular,  or  of  the  successive  periods 
at  which  the  different  fossiliferous  formations  have  been  deposited. 

Now  there  are  three  principal  tests  by  which  we  determine 
the  age  of  a  given  set  of  strata ;  first,  superposition ;  secondly, 
mineral  character;  and,  thirdly,  organic  remains.  Some  aid 
can  occasionally  be  derived  from  a  fourth  kind  of  proof,  namely, 
the  fact  of  one  deposit  including  in  it  fragments  of  a  preexisting 
rock,  which  last  may  thus  be  shown,  even  in  the  absence  of  all 
other  evidence,  to  be  the  older  of  the  two. 

Superposition. — The  first  and  principal  test  of  the  age  of  one 
aqueous  deposit,  as  compared  to  another,  is  relative  position.  It 
has  been  already  stated,  that  where  the  strata  are  horizontal,  the 
bed  which  lies  uppermost  is  the  newest  of  the  whole,  and  that 
which  lies  at  the  bottom  the  most  ancient.  So,  of  a  series  of 
sedimentary  formations,  they  are  like  volumes  of  history,  in 
which  each  writer  has  recorded  the  annals  of  his  own  times, 
and  then  laid  down  the  book,  with  the  last  written  page  upper- 
most, upon  the  volume  in  which  the  events  of  the  era  immedi- 
ately preceding  were  commemorated.  In  this  manner  a  lofty 
pile  of  chronicles  is  at  length  accumulated;  and  they  are  so 
arranged  as  to  indicate,  by  their  position  alone,  the  order  in 
which  the  events  recorded  in  them  have  occurred. 

In  regard  to  the  crust  of  the  earth,  however,  there  are  some 
regions  where,  as  the  student  has  already  been  informed,  the 
beds  have  been  disturbed,  and  sometimes  reversed.  (See  pp.  73, 
74.)  But  the  experienced  geologist  will  not  be  deceived  by 


160  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Tests  of  the  different  Ages  of  Aqueous  Rocks. 

these  exceptional  cases.  When  he  finds  that  the  strata  are  frac- 
tured, curved,  inclined,  or  vertical,  he  knows  that  the  original 
order  of  superposition  must  be  doubtful,  and  he  will  endeavour 
to  find  sections  in  some  neighbouring  district  where  the  strata 
are  horizontal,  or  only  slightly  inclined.  Here  it  is  impossible 
that  they  can  have  been  extensively  thrown  over  and  turned 
upside  down,  for  such  a  derangement  cannot  have  taken  place 
throughout  a  wide  area  without  leaving  manifest  signs  of  dis- 
placement and  dislocation. 

Mineral  character. — The  same  rocks  may  often  be  observed 
to  retain  for  miles,  or  even  hundreds  of  miles,  the  same  mineral 
peculiarities,  if  we  follow  them  in  the  direction  of  the  planes  of 
stratification.  But  this  uniformity  ceases  almost  immediately,  if 
we  pursue  them  in  an  opposite  direction.  In  that  case  we  can 
scarcely  ever  penetrate  a  stratified  mass  for  a  few  hundred  yards, 
much  less  several  miles,  without  beholding  a  succession  of  ex- 
tremely dissimilar  calcareous,  argillaceous,  and  siliceous  rocks. 
These  phenomena  lead  to  the  conclusion,  that  rivers  and  currents 
have  dispersed  the  same  sediment  over  wide  areas  at  one  period, 
but  at  successive  periods  have  been  charged,  in  the  same  region, 
with  very  different  kinds  of  matter.  The  first  observers  were 
so  astonished  at  the  vast  -spaces  over  which  they  were  able  to 
follow  the  same  homogeneous  rocks  in  a  horizontal  direction, 
that  they  came  hastily  to  the  opinion,  that  the  whole  globe  had 
been  environed  by  a  succession  of  distinct  aqueous  formations, 
disposed  round  the  nucleus  of  the  planet,  like  the  concentric 
coats  of  an  onion.  But  although,  in  fact,  some  formations  may 
be  continuous  over  districts  as  large  as  half  of  Europe,  or  even 
more,  yet  most  of  them  either  terminate  wholly  within  narrower 
limits,  or  soon  change  their  lithological  character.  Sometimes 
they  thin  out  gradually,  as  if  the  supply  of  sediment  had  failed 
in  that  direction,  or  they  come  abruptly  to  an  end,  as  if  we  had 
arrived  at  the  borders  of  the  ancient  sea  or  lake  which  served 
as  their  receptacle.  It  no  less  frequently  happens  that  they  vary 
in  mineral  aspect  and  composition,  as  we  pursue  them  horizon- 
tally. For  example,' we  trace  a  limestone  for  a  hundred  miles, 
until  it  becomes  more  arenaceous,  and  finally  passes  into  sand, 
or  sandstone.  We  may  then  follow  this  sandstone,  already  proved 
by  its  continuity  to  be  of  the  same  age,  throughout  another  dis- 
trict a  hundred  miles  or  more  in  length. 

Organic  remains. — This  character  must  be  used  as  a  crite- 
rion of  the  age  of  a  formation,  or  of  the  contemporaneous  origin 
of  two  deposits  in  distant  places,  under  very  much  the  same 
restrictions  as  the  test  of  mineral  composition. 

First,  the  same  fossils  may  be  traced  over  wide  regions,  if 


PART  II.     CHAPTER  XIII.  161 

Tests  of  the  different  Ages  of  Aqueous  Roctfs. 

we  examine  strata  in  the  direction  of  their  planes,  although  by 
no  means  for  indefinite  distances.  This  might  have  been  ex- 
pected ;  for  although  many  species  of  animals  and  plants  have 
a  wide  geographical  range,  yet  each  species  generally  inhabits  a 
small  part  only  of  the  entire  globe,  and  is  often  incapable  of 
existing  in  other  regions.  But,  in  those  cases  where  the  fossils 
vary,  the  mineral  character  of  the  rock  often  remains  constant  ; 
and,  on  the  other  hand,  the  fossils  are  sometimes  uniform  through- 
out spaces  where  the  lithological  nature  of  the  rock  is  variable. 
In  this  manner  we  are  frequently  enabled  to  prove  the  contem- 
poraneous origin  of  the  same  formation  by  one  test,  when  the 
other  fails. 

Secondly,  while  the  same  fossils  prevail  in  a  particular  set  of 
strata  for  hundreds  of  miles  in  a  horizontal  direction,  we  seldom 
meet  with  the  same  remains  for  many  fathoms,  and  scarcely 
ever  for  several  hundred  yards,  in  a  vertical  line,  or  a  line  trans- 
verse to  the  strata.  This  fact  has  now  been  verified  in  almost 
all  parts  of  the  globe,  and  has  led  to  a  conviction,  that  at  suc- 
cessive periods  of  the  past,  the  same  area  of  land  and  water  has 
been  inhabited  by  species  of  animals  and  plants  as  distinct  as 
those  which  now  people  the  antipodes,  or  which  now  coexist  in 
the  arctic,  temperate,  and  tropical  zones.  It  appears,  that  from 
the  remotest  periods  there  has  been  ever  a  coming  in  of  new 
organic  forms,  and  an  extinction  of  those  which  pre-existed  on 
the  earth ;  some  species  having  endured  for  a  longer,  others  for 
a  shorter  time ;  but  none  having  ever  re-appeared  after  once  dy- 
ing out.  The  law  which  has  governed  the  creation  and  extinc- 
tion of  species  seems  to  be  expressed  in  the  verse  of  the  poet, 

Natura  il  fece  e  poi  ruppe  la  starapa.— Ariosto. 
Nature  made  it,  and  then  broke  the  die. 

And  this  circumstance  it  is,  which  confers  on  fossils  their  highest 
value  as  chronological  tests,  giving  to  each  of  them,  in  the  eyes 
of  the  geologist,  that  authority  which  belongs  to  contemporary 
medals  in  history. 

The  same  cannot  be  said  of  each  peculiar  variety  of  rock ; — 
for  some  of  these,  as  red  marl  and  red  sandstone,  for  example, 
may  occur  at  once  at  the  top,  bottom,  and  middle  of  the  entire 
sedimentary  series ;  exhibiting  in  each  position  so  perfect  an 
identity  of  mineral  aspect  as  to  be  undistinguishable.  Such 
exact  "repetitions,  however,  of  the  same  mixtures  of  sediment 
have  not  often  occurred,  at  distant  periods,  in  precisely  the  same 
parts  of  the  globe ;  and  even  where  this  has  happened,  we  may 
usually  avoid  confounding  together  the  monuments  of  remote 
eras,  by  the  aid  of  fossils  and  relative  position. 


162 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Chronological  Classification  of  Aqueous  Rocks. 


Test  by  included  fragments  of  old  rocks. — It  was  stated, 
that  independent  proof  may  sometimes  be  obtained  of  the  rela- 
tive date  of  two  formations,  by  fragments  of  an  older  rock  being 
included  in  a  newer  one.  This  evidence  may  sometimes  be  of 
great  use,  where  a  geologist  is  at  a  loss  to  determine  the  relative 
age  of  two  formations,  from  want  of  clear  sections  exhibiting 
their  true  order  of  position,  or  because  the  strata  of  each  group 
are  vertical.  In  such  cases  we  sometimes  discover  that  the 
more  modern  rock  has  been  in  part  derived  from  the  degradation 
of  the  older.  Thus,  for  example,  we  may  find  chalk  with  flints ; 
and,  in  another  part  of  the  same  country,  a  distinct  series,  con- 
sisting of  alternations  of  clay,  sand,  and  pebbles.  If  some  of 
these  pebbles  consist  of  flints,  with  fossil  shells  of  the  same  spe- 
cies as  those  in  the  chalk,  we  may  confidently  infer  that  the  chalk 
is  the  oldest  of  the  two  formations. 

The  number  of  groups  into  which  the  fossiliferous  strata  may 
be  separated,  is  more  or  less,  according  to  the  views  of  classifi- 
cation which  different  geologists  entertain;  but  when  we  have 
adopted  a  certain  system  of  arrangement,  we  immediately  find 
that  a  few  only  of  the  entire  series  of  groups  occur  one  upon 
the  other  in  any  single  section  or  district. 

The  thinning  out  of  individual  strata  was  before  described 
(p.  31.).  But  let  the  annexed  diagram  represent  seven  fossili- 

Fig.  127. 


ferous  groups,  instead  of  as  many  strata.  It  will  then  be  seen 
that  in  the  middle  all  the  superimposed  formations  are  present ; 
but  in  consequence  of  some  of  them  thinning  out,  No.  2.  and 
No.  5.  are  absent  at  one  extremity  of  the  section,  and  No.  4.  at 
the  other. 

If  the  reader  consults  the  Frontispiece,  he  will  see,  that  as  the 
strata  A  rest  unconformably  upon  the  older  groups,  a,  ft,  c,  e,f, 
g,  we  should  meet  with  a  very  different  succession  in  a  vertical 
section  exposed  at  different  places ;  in  one  spot  A  lying  imme- 
diately on  c,  in  another  on  g,  and  so  forth.  Now  here  the  dif- 
ference has  been  partly  occasioned  by  denudation ;  the  forma- 
tions a,  ft,  for  instance,  once  extended  much  farther  to  the  left, 
and  but  for  denudation  would  have  been  everywhere  interposed 
between  A  and  the  rocks  e,  f,  g.  In  many  instances  the  entire 
absence  of  one  or  more  formations  of  intervening  periods  be- 
tween two  groups,  such  as  A  and  c,  (see  Frontispiece,)  arises, 


PART  II.    CHAPTER  XIII.  163 


Chronological  Classification  of  Aqueous  Rocks. 


not  from  the  destruction  of  what  once  existed,  by  denudation,  but 
because  no  strata  of  an  intermediate  age  were  ever  deposited  on  c. 
They  were  not  formed  at  that  place,  either  because  the  region 
was  dry  land  during  the  interval,  or  because  it  was  part  of  a  sea 
or  lake  to  which  no  sediment  was  carried. 

In  order,  therefore,  to  establish  a  chronological  succession  of 
fossil iferous  groups,  a  geologist  must  begin  with  a  single  section, 
in  which  several  sets  of  strata  lie  one  upon  the  other.  He  must 
then  trace  these  formations,  by  attention  to  their  mineral  char- 
acter and  fossils,  continuously,  as  far  as  possible,  from  the  start- 
ing point.  As  often  as  he  meets  with  new  groups,  he  must  ascer- 
tain by  superposition  their  age  relatively  to  those  first  examined, 
and  thus  learn  how  to  intercalate  them  in  a  tabular  arrangement 
of  the  whole. 

By  this  means  the  German,  French,  and  English  geologists 
have  determined  the  succession  of  strata  throughout  a  great  part 
of  Europe,  and  have  adopted  pretty  generally  the  following 
groups,  almost  all  of  which  have  their  representatives  in  the 
British  Islands. 

Groups  of  Fossiliferous  Strata  observed  in  Western  Europe, 
arranged  in  what  is  termed  a  descending  series,  or  beginning 
with  the  newest. 


I. 

Newer  Pliocene. 

2. 
3. 

Older  Pliocene. 
Miocene. 

Tertiary  or  Supracretaceous.* 

4. 

Eocene. 

5. 

Chalk. 

§ 

6. 

Greensand. 

7. 

Wealden. 

8. 

Upper  Oolite. 

9. 

Middle  Oolite. 

10. 

Lower  Oolite. 

11. 

Lias. 

>  Secondary. 

12. 

Upper  New  Red  sandstone  and 

Muschelkalk. 

13. 

Lower  New  Red  and  Magnesian 

limestone. 

14. 

Coal. 

15. 

Old  Red  sandstone. 

16. 
17. 

Upper  Silurian.                               "1 
Lower  Silurian.                               (Primary  fossiliferous  (or  transition  of 

18. 

Cambrian  and  older  fossiliferous  j      some  authors). 

strata.                                        J 

A  glance  at  the  above  table  will  show  that  the  three  great 
sections  called  primary  fossiliferous,  secondary,  and  tertiary,  are 
by  no  means  of  equivalent  importance,  if  the  eighteen  subordi- 
nate groups  comprise  monuments  relating  to  equal  portions  of 

*  For  tertiary,  Mr.  De  la  Beche  has  used  the  term  "  supracretaceous,"  a  name 
implying  that  the  strata  so  called  are  superior  in  position  to  the  chalk. 


164      LYELL'S  ELEMENTS  OF  GEOLOGY. 


Chronological  Classification  of  Aqueous  Rocks. 


past  time,  or  of  the  earth's  history.  But  this  we  cannot  assert ; 
but  merely  know  that  they  each  relate  to  successive  periods, 
during  which  certain  animals  and  plants,  for  the  most  part  pecu- 
liar to  that  era,  flourished,  and  during  which  different  kinds  of 
sediment  were  deposited  in  the  space  now  occupied  by  Europe. 

If  we  were  disposed,  on  palseontologieal  grounds,  to  divide  the 
entire  fossiliferous  series  into  a  few  groups,  less  numerous  than 
those  in  the  above  table,  and  more  nearly  co-ordinate  in  value 
than  the  sections  called  primary,  secondary,  and  tertiary,  we 
might,  perhaps,  adopt  the  six  following  groups  or  periods.*  At 
the  same  time  I  may  observe,  that  in  the  present  state  of  the 
science,  when  we  have  not  yet  compared  the  evidence  derivable 
from  all  classes  of  fossils,  not  even  those  most  generally  distri- 
buted, such  as  shells,  corals,  and  fish,  such  generalizations  are 
premature,  and  can  only  be  regarded  as  conjectural  schemes  for 
the  founding  of  large  natural  groups. 

1 .  Tertiary from  the  Newer  Pliocene  to  the  Eocene  inclusive. 

2.  Cretaceous from  the  Chalk  to  the  Wealden  inclusive. 

3.  Oolitic from  the  Oolite  to  the  Lias  inclusive. 

A    TT         AT       o  A  $  including  the  Keuper,  Muschelkalk,  and  Bunter 

4.  Upper  New  Red J     Sandstein  of  the  Germans. 

5.  Lower  New  Red    and  (  including  Magnesian  Limestone  (Zechstein),  Coal, 

Carboniferous  .....'..  (     and  Old  Red  sandstone. 

6.  Primary  fossiliferous  ...  j  ^S^SS^?^  tOthe  °WeSt  fossiliferous 

The  limits  of  this  volume  will  not  allow  of  a  full  description, 
even  of  the  leading  features  of  all  the  formations  enumerated  in 
the  above  tables ;  but  I  shall  briefly  advert  to  each  of  them  in 
chronological  order,  as  they  will  afford  illustrations  of  the  rules 
of  classification,  the  tests  of  relative  age,  and  the  mode  of  deriving 
information  from  geological  monuments  respecting  the  former 
history  of  the  earth  and  its  inhabitants. 

Tertiary  formations. — TJiese  strata,  as  we  have  seen,  were 
so  called  because,  when  first  discovered,  they  were  observed  to 
be  of  a  date  posterior  to  the  chalk,  which  had  long  been  regarded 
as  the  last  or  uppermost  of  the  secondary  formations.  It  was 
remarked,  that  in  France,  Italy,  Germany,  and  England,  the 
tertiary  deposits  occupied  a  position,  in  reference  to  all  older 
rocks,  like  that  of  the  waters  of  lakes,  inland  seas,  and  gulfs  in 
relation  to  a  continent,  being  often,  like  such  waters,  of  great 
depth,  though  of  limited  area,  and  frequently  occurring  in  de- 
tached and  isolated  patches.  The  strata  were  for  the  most  part 

*  Palaeontology  is  the  science  which  treats  of  fossil  remains,  both  animal  and 
vegetable.  Elym.  ira\aios,  palaios,  ancient,  oira,  onto,  beings,  and  Xoyoj,  logos, 
a  discourse. 


PART  II.     CHAPTER  XIII.  105 

Classification  of  the  Tertiary  Formations. 

horizontal,  but  usually  surrounded  by  older  rocks,  of  which  the 
beds  were  highly  inclined  or  vertical. 

On  comparing  together  the  fossils  of  the  aqueous  formations 
in  general,  especially  the  testacea,  which  are  the  most  abundant 
and  best  preserved  of  all,  it  appears  that  those  of  the  primary 
fossiliferous  rocks  depart  most  widely  in  form  and  structure  from 
the  type  of  the  living  creation,  those  of  the  secondary  less  wide- 
ly, and  the  tertiary  least  of  all.  In  like  manner,  if  we  divide 
the  tertiary  deposits  into  four  principal  groups,  and  then  compare 
the  fossil  shells  which  they  contain  with  the  testacea  now  living 
in  the  nearest  seas  in  the  same  latitudes,  we  find  that  the  shells 
of  the  oldest  strata  have  much  less  resemblance,  on  the  whole, 
to  the  fauna  of  the  neighbouring  seas,  than  those  of  the  newest 
group.  In  a  word,  in  proportion  as  the  age  of  a  tertiary  forma- 
tion is  more  modern,  so  also  is  the  resemblance  greater  of  its 
fossil  shells  to  the  testaceous  fauna  of  the  actual  seas. 

Having  observed  the  prevalence  of  this  change  of  character 
in  the  tertiary  strata  of  France  and  Italy,  in  1828,  I  conceived 
the  idea  of  classing  the  whole  series  of  tertiary  strata  into  four 
groups,  endeavouring  to  find  characters  for  each  expressive  of 
their  different  degrees  of  affinity  to  the  living  fauna.  I  hoped 
that  an  estimate  of  this  varying  relation  to  the  fauna  of  the 
existing  seas  might  be  obtained  by  determining  the  proportional 
number  of  shells  identical  with  living  species  which  belonged  to 
each  group.  With  this  view,  I  obtained  information  respecting 
the  specific  identity  of  many  tertiary  and  recent  shells  from  seve- 
ral Italian  naturalists  ;  and  among  others,  from  Professors  Bo- 
nelli,  Guidotti,  and  Costa. 

I  have  explained  at  length,  in  the  Principles  of  Geology,  the 
opinions  which  were  at  that  time  generally  entertained  respecting 
the  classification  of  tertiary  formations,  and  the  observations 
which  led  me,  in  1828,  to  divide  them  into  four  groups,  by  refer- 
ence not  only  to  their  geological  position,  but  also  to  the  propor- 
tional number  of  recent  species  found  fossil  in  each.  I  have  also 
there  stated,  that  having,  in  1829,  become  acquainted  with  M. 
Deshayes,  of  Paris,  I  learnt  from  him  that  he  had  arrived,  by  in- 
dependent researches,  and  by  the  study  of  a  large  collection  of 
fossil  and  recent  shells,  at  very  similar  views.  At  my  request 
he  drew  up,  in  a  tabular  form,  lists  of  all  the  shells  known  to 
him  to  occur  both  in  some  tertiary  formation  and  in  a  living  state, 
for  the  express  purpose  of  ascertaining  the  proportional  number 
of  fossil  species  identical  with  the  recent  which  characterized  the 
successive  group  ;  and  this  table  was  published  by  me  in  1833.* 

*  See  Principles  of  Geology,  vol.  iii.  1st  ed. 


166  LYELI/S-  ELEMENTS  OF  GEOLOGY.       ^ 

Cfossification  of  the  Tertiary  Formations. 

The  number  of  tertiary  fossil  shells  examined  by  M.  Deshayes 
was  about  3000 ;  and  the  recent  species  with  which  they  had 
been  compared,  about  5000.  The  result  at  which  that  naturalist 
arrived  was,  that  in  the  oldest  tertiary  deposits,  such  as  those 
found  near  London  and  Paris,  there  were  about  3^  per  cent,  of 
species  of  fossil  shells  identical  with  recent  species ;  in  the  next, 
or  middle  tertiary  period,  to  which  certain  strata  on  the  Loire 
and  Gironde,  in  France,  belonged,  about  17  per  cent. ;  and  in 
the  deposits  of  a  third,  or  newer  era,  embracing  those  of  the 
Subapennine  hills,  from  35  to  50  per  cent.  In  formations  still 
more  modern,  some  of  which  I  had  particularly  studied  in  Sicily, 
where  they  attain  a  vast  thickness  and  elevation  above  the  sea, 
the  number  of  species  identical  with  those  now  living  was  from 
90  to  95  per  cent.  For  the  sake  of  clearness  and  brevity,  I  pro- 
posed to  give  short  technical  names  to  these  four  groups,  or  the 
periods  to  which  they  respectively  belonged.  I  called  the  first  or 
oldest  of  them  Eocene,  the  second  Miocene,  the  third  Older  Plio- 
cene, and  the  last  or  fourth,  Newer  Pliocene.  The  first  of  the 
above  terms,  Eocene,  is  derived  from  »;wj,  eos,  dawn,  and  seawoj, 
cainos,  recent,  because  the  fossil  shells  of  this  period  contain  an 
extremely  small  proportion  of  living  species,  which  may  be  look- 
ed upon  as  indicating  the  dawn  of  the  recent  or  existing  state 
of  the  testaceous  fauna. 

The  other  terms,  Miocene  and  Pliocene,  are  comparative ;  the 
first  meaning  less  recent,  (from  pt tw,  meion,  less,  and  xawo$,  cainos, 
recent,)  and  the  other  more  recent,  (from  ttteiov,  pleion,  more,  and 
jcaw/05,  cainos,  recent,)  they  express  the  more  or  less  near  ap- 
proach which  the  deposits  of  these  eras,  when  contrasted  with 
each  other,  make  to  the  existing  creation,  at  least  so  far  as  the 
mollusca  are  concerned.  It  may  assist  the  memory  of  students 
to  remind  them,  that  the  Miocene  contain  a  minor  proportion, 
and  PZiocene  a  comparative  pZurality  of  recent  species ;  and 
that  the  greater  number  of  recent  species  always  implies  the 
more  modern  origin  of  the  strata. 

Two  subjects  of  discussion  have  arisen  respecting  the  tables 
above  alluded  to ;  first,  whether  the  fossil  shells  were,  upon  the 
whole,  correctly  identified  with  recent  species  by  M.  Deshayes ; 
secondly,  whether  such  a  per-centage  of  recent  species  occurring 
fossil  in  particular  groups,  affords  the  best  criterion  for  estimat- 
ing the  relation  of  each  fossil  fauna  to  the  living  creation. 

Now  in  regard  to  the  per-centage  test,  its  application  must 
evidently  depend  on  the  extent  to  which  conchologists  are  agreed 
in  their  determination  of  species.  In  every  branch  of  natural 
history  there  is  always  some  difference  of  opinion  as  to  certain 
species  which  are  variable  in  their  characters,  and  seem  to  pass 


PART  II.     CHAPTE 


Classification  of  the  Tertiary  Forma 


by  imperceptible  gradations  into  other  forms,  considered!  fty  many 
zoologists  and  botanists  as  entitled  to  rank  as  distinct  species. — 
The  difficulty  of  defining  the  limits  in  such  cases  is  not  greater, 
perhaps,  in  conchology  than  in  other  departments  ;  but  it  happens 
that  this  science  has  advanced  very  rapidly  since  the  year  1830, 
when  M.  Deshayes  drew  up  the  tables  published  in  the  Principles 
of  Geology.  In  that  year  he  had  it  in  his  power  to  refer  to  no 
more  than  5000  species  of  recent  shells  then  in  Paris ;  but  the 
number  of  species  now  in  the  public  and  private  collections  of 
Europe  has  increased  to  between  8000  and  9000 ;  and,  what  is 
of  no  less  consequence,  individuals  of  species  which  before  that 
time  were  extremely  rare,  have  been  supplied  in  abundance. 
Fossil  shells  also  have  been  collected  with  equal  zeal  and  suc- 
cess ;  and  thus  the  facility  of  discriminating  nice  distinctions  in 
closely  allied  species,  or  of  deciding  which  characters  are  con- 
stant and  which  variable,  has  been  greatly  promoted ;  and  the 
study  of  these  more  ample  data  has  led  all  conchologists  to 
separate  many  species,  both  fossil  and  recent  shells,  which  before 
they  had  confounded  together. 

In  consequence  of  the  changes  of  opinion  brought  about  by 
these  additions  to  our  knowledge,  it  has  become  necessary  not 
only  to  examine  all  the  newly  discovered  fossil  and  recent  tes- 
tacea,  but  also  to  reconsider  all  the  species  previously  known. 
As  this  laborious  task  has  not  yet  been  executed  by  M.  Deshayes, 
engaged  as  he  is  in  other  scientific  Jabours,  I  am  unable  at  present 
to  offer  to  the  reader  the  improved  results  which  the  revision  of 
the  tables  drawn  up  in  1830  would  afford.  In  the  mean  time  I 
have  obtained  the  aid  of  several  eminent  conchologists,  arid  in 
particular  of  Dr.  Beck,  of  Copenhagen,  in  comparing  a  great 
number  of  the  recent  and  fossil  shells  which  had  been  identified. 
By  this  investigation  I  have  come  to  the  conclusion*  that  the  per- 
centage of  recent  species  in  a  fossil  state  is  decidedly  less,  espe- 
cially in  the  older  tertiary  strata,  than  was  indicated  in  the  list 
published  in  1833.  A  large  number,  in  particular,  of  the  forty- 
two  species  of  ^Eocene  testacea,  to  which  the  names  of  recent 
shells  were  given  in  the  tables,  cannot  be  considered  as  identical, 
if  we  adopt  the  same  standard  of  specific  distinctions  as  is  recog- 
nized in  the  new  edition  of  Lamarck's  conchology,  edited  by  M. 
Deshayes  himself,  in  1836. 

But  although  many  corrections  are  indispensable,  and  the 
proportion  of  recent  species  found  fossil  in  the  Eocene,  Miocene, 
and  older  Pliocene  strata  may  be  considerably  less  than  was  at 
first  supposed,  we  have  no  reason  on  this  account  to  feel  dis- 
couraged in  an  attempt  to  found  the  classification  and  nomen- 
clature of  the  tertiary  periods  on  the  great  principle  before 


168  LYELL'S  ELEMENTS  OF  GEOLOGY. 


Classification  of  the  Tertiary  Formations. 


explained ;  namely,  the  comparative  resemblance  of  the  testa- 
ceous fauna  of  each  period  to  that  of  the  neighbouring  seas. 
There  can  be  no  cabalistic  virtue  in  such  numbers  as  3.  17.  or 
40.,  which  were  at  first  imagined  to  express  correctly  the  pro- 
portional number  of  identical  species  in  three  of  the  tertiary 
periods ;  but  until  the  time  arrives  when  we  can  obtain  the  gene- 
ral acquiescence  of  conchologists  as  to  the  real  proportional 
numbers,  we  must  endeavour  to  find  some  readier  method  of 
estimating  the  relation  of  one  fauna  to  another ;  a  method  not 
involving  the  question  of  the  identity  or  non-identity  of  every 
fossil  with  some  known  recent  species. 

Now,  it  has  been  suggested  by  Dr.  Beck  that,  in  order  to  form 
such  an  estimate  of  the  comparative  resemblance  of  the  faunas 
of  different  eras,  we  may  follow  the  same  plan  as  would  enable 
us  to  appreciate  the  amount  of  agreement  or  discrepancy  between 
the  faunas  now  existing  in  two  distinct  geographical  regions. 

It  is  well  known  that,  although  nearly  all  the  species  of  mol- 
lusca  inhabiting  the  temperate  zones  on  each  side  of  the  equator 
are  distinct,  yet  the  whole  assemblage  of  species  in  one  of  these 
zones  bears  a  striking  analogy  to  that  in  the  other,  and  differs  in 
a  corresponding  manner  from  the  tropical  and  arctic  faunas.  By 
what  language  can  the  zoologist  express  such  points  of  agree- 
ment or  disagreement,  where  the  species  are  admitted  to  be  dis- 
tinct? 

In  such  cases  it  is  necessary  to  mark  the  relative  abundance 
in  the  two  regions  compared  of  certain  families,  genera,  and 
sections  of  genera ;  the  entire  absence  of  some  of  these,  the 
comparative  strength  of  others,  this  strength  being  sometimes 
represented  by  the  numbers  of  species,  sometimes  by  the  great 
abundance  and  size  of  the  individuals  of  certain  species.  It  is, 
moreover,  important  to  estimate  the  total  number  of  species 
inhabiting  a  given  area;  and  also  the  average  proportion  of 
species  to  genera,  as  this  differs  materially  according  to  climate. 
Thus,  if  we  adopt  comprehensive  genera  like  those  of  Lamarck, 
we  shall  find,  according  to  Dr.  Beck,  that,  upon^an  average,  there 
are  in  arctic  latitudes  nearly  as  many  genera  as  species ;  in  the 
temperate  regions,  about  three  or  four  species  to  a  genus ;  in  the 
tropical,  five  or  six  species  to  a  genus. 

The  method  of  which  the  above  sketch  conveys  but  a  faint 
outline,  is  the  more  easy  of  application  to  the  tertiary  deposits 
of  Europe,  because  the  conchological  fauna  of  the  Eocene  period 
indicates  a  tropical  climate;  that  of  the  Miocene  strata,  a  climate 
bordering  on  the  tropics;  and  that  of  the  Older  and  Newer 
Pliocene  deposits,  a  climate  much  more  closely  approaching  to, 
if  not  the  same  as,  that  of  the  seas  in  corresponding  latitudes. 


PART  II.     CHAPTER  XIV.  169 


Recent  and  Newer  Pliocene  Strata. 


Although  I  cannot  enter  in  this  work  into  farther  details,  it 
may  be  stated  that,  if  we  compare  tertiary  formations  on  this 
principle,  the  nomenclature  above  proposed  will  not  be  inappro- 
priate ;  for  the  fauna  of  the  older,  or  Eocene  tertiary  formations 
is  still  the  first  in  the  order  of  time  in  which  there  is  an  assem- 
blage of  testacea  like  that  of  the  present  ocean  between  the  tro- 
pics ;  and  in  this  period  a  small  proportion  of  mollusca  are  un- 
distinguishable  from  living  species ;  whereas  at  the  opposite  ex- 
treme of  the  series,  or  in  the  Newer  Pliocene  deposits,  ail  con- 
chologists  agree  that  the  marine  shells  are  all,  or  nearly  all, 
identical  with  those  now  inhabiting  the  nearest  seas.  As  to  the 
Miocene  and  Older  Pliocene  groups,  the  terms  less  and  more  will 
always  express  correctly  the  different  degrees  of  analogy  which 
their  fossils  bear  to  the  assemblage  of  living  species  in  similar 
latitudes. 

But  it  should  never  be  forgotten,  that,  as  the  extinct  species 
preponderate  in  all  groups,  with  the  exception  of  the  Newer  Pli- 
ocene, it  is  from  their  characters  that  we  derive  the  distinguish- 
ing feature  in  the  palaeontology  of  each  period.  The  relative 
approach  which  the  shells  may  make  to  the  living  fauna  affords 
a  useful  and  interesting  term  of  comparison ;  but  it  is  one  fea- 
ture only,  and  by  no  means  the  most  prominent  one,  in  the  or- 
ganic remains  of  successive  periods. 


CHAPTER  XIV. 

RECENT    AND    TERTIARY    FORMATIONS. 

How  to  distinguish  Recent  from  Tertiary  strata — Recent  and  Newer  Pliocene 
strata  near  Naples  —  near  Stockholm  and  Christiania  —  in  South  America,  on 
coasts  of  Chili  and  Peru  —  Rocks  of  recent  period,  with  human  skeleton,  in 
Guadaloupe— Shells  of  living  species,  with  extinct  mammalia,  in  loess  of  the 
Rhine — Recent  and  Newer  Pliocene  deposits  in  England— Older  Pliocene  strata 
in  England  — Crag  — Red  and  Coralline  crag  —  their  fossils  in  part  distinct  — 
their  strata  uncomfortable  —  belong  to  the  same  period  —  London  clay — Its 
shells  and  fish  imply  a  tropical  climate  — Tertiary  mammalia  —  Fossil  quadru- 
mana. 

Recent  and  Newer  Pliocene  strata.- — If  we  begin  with  the  his- 
tory of  the  more  modern  aqueous  formations,  and  then  pass  on 
to  the  more  ancient,  the  first  strata  which  present  themselves  are 
those  termed,  in  the  last  chapter,  the  Newer  Pliocene.  But  in 


170      LYELL'S  ELEMENTS  OF  GEOLOGY. 


Recent  and  Newer  Pliocene  Strata. 


what  manner  shall  we  define  the  limits  between  this  group  and 
those  fossiliferous  deposits  which  are  now  in  progress,  or  which 
have  accumulated  under  water  since  the  globe  was  inhabited  by 
man?  The  strata  last  mentioned,  namely,  those  of  the  human 
period,  I  shall  call  Recent,  distinguishing  them  from  the  most 
modern  tertiary  formations.  Strata  may  be  proved  to  belong  to 
the  Recent  period  by  our  finding  in  them  the  bones  of  man  in  a 
fossil  state,  that  is  to  say,  imbedded  in  them  by  natural  causes  ; 
or  we  may  recognize  them  by  their  containing  articles  fabricated 
by  the  hands  of  man,  or  by  showing  that  such  deposits  did  not 
exist  in  the  place  where  we  now  observe  them  at  a  given  period 
of  the  past  when  man  existed,  so  that  they  must  be  of  subsequent 
origin.  In  general  all  recent  formations  lie  hidden  from  our  sight 
beneath  the  waters  of  lakes  and  seas  ;  but  we  may  examine  them 
wherever  these  lakes  or  seas  have  been  partially  converted  into 
land,  as  in  the  deltas  of  rivers,  or  where  the  submerged  ground 
has  been  heaved  up  by  subterranean  movements,  and  laid  dry. 

Thus  at  Puzzuoli,  near  Naples,  marine  strata  are  seen  contain- 
ing fragments  of  sculpture,  pottery,  and  the  remains  of  buildings, 
together  with  innumerable  shells  retaining  in  part  their  colour, 
and  of  the  same  species  as  those  now  inhabiting  the  Mediterranean. 
The  uppermost  of  these  beds  is  about  twenty  feet  above  the  level 
of  the  sea.  Their  emergence  can  be  proved  to  have  taken  place 
since  the  beginning  of  the  sixteenth  century.*  But  the  hills  at 
the  base  of  which  these  strata  have  been  deposited,  and  those  of 
the  interior  of  the  adjacent  country  round  Naples,  some  of  which 
rise  to  the  height  of  1500  feet  above  the  sea,  are  formed  of  hori- 
zontal strata  of  the  Newer  Pliocene  period ;  that  is  to  say,  the 
marine  shells  observed  in  them  are  of  living  species,  and  yet  are 
not  accompanied  by  any  remains  of  man  or  his  works.  Had 
such  been  discovered,  it  would  have  afforded  to  the  antiquary 
and  geologist  matter  of  great  surprise,  since  it  would  have  shown 
that  man  was  an  inhabitant  of  that"  part  of  the  globe,  while  the 
materials  composing  the  present  hills  and  plains  of  Campania 
were  still  in  the  progress  of  deposition  at  the  bottom  of  the  sea ; 
whereas  we  know  that  for  nearly  3000  years,  or  from  the  times 
of  the  earliest  Greek  colonists,  no  extensive  revolution  in  the 
physical  geography  of  that  part  of  Italy  has  occurred. 

In  Sweden,  analogous  phenomena  have  been  observed.  Near 
Stockholm,  for  example,  when  the  canal  of  Sodertelje  was  dug, 
horizontal  beds  of  sand,  loam,  and  marl  were  passed  through,  in 
some  of  which  the  same  peculiar  assemblage  of  testacea  which 
now  live  in  the  Baltic  were  found.  Mingled  with  these,  at  different 

*  See  Principles  of  Geology,  Index,  "  Serapis." 


PART  II.     CHAPTER  XIV.  171 

Recent  and  Newer  Pliocene  Strata. 

depths,  were  detected  various  works  of  art  implying  a  rude  state 
of  civilization,  and  some  vessels  built  before  the  introduction  of 
iron.  These  vessels  and  implements  must  have  sunk  to  the 
bottom  of  an  arm  of  the  sea,  afterwards  filled  up  with  sand  and 
loam  including  marine  shells,  and  the  whole  must  then  have  been 
upraised ;  so  that  the  upper  beds  became  sixty  feet  higher  than  * 
the  surface  of  the  Baltic.  There  are,  however,  in  the  neighbour- 
hood of  these  formations,  others  precisely  similar  in  mineral 
composition  and  testaceous  remains,  which  ascend  to  the  height 
of  between  100  and  200  feet,  in  which  no  vestige  of  human  art 
has  been  seen.  Similar  deposits  reach  an  elevation  of  500  and 
even  600  feet  in  Norway,  as  in  the  neighbourhood  of  Christiania, 
where  they  have  usually  been  described  as  raised  beaches,  but 
are,  in  fact,  strata  of  clay,  sand,  and  marl,  often  many  hundred 
feet  thick,  which  cover  the  inland  country  far  ancj  wide,  filling 
valleys  and  deep  depressions  in  the  granite  gneiss,  and  primary 
fossiliferous  rocks,  just  as  the  tertiary  formations  of  England  and 
France  rest  upon  the  chalk,  or  fill  depressions  in  it. 

All  conchologists  are  agreed  that  the  shells  of  the  deposits 
above  mentioned  are  nearly  all,  perhaps  all,  absolutely  identical 
with  those  now  peopling  the  contiguous  ocean ;  so  that,  in  the 
absence  of  any  evidence  of  their  being  Recent,  we  must  regard 
them  as  Newer  Pliocene  formations. 

Along  the  western  shores  of  South  America,  RecenJ  and 
Newer  Pliocene  strata  have  in  like  manner  been  brought  to  light. 
These  often  consist  of  enormous  masses  of  shells,  similar  to  those 
now  swarming  in  the  Pacific.  In  one  bed  of  this  kind,  in  the 
island  of  San  Lorenzo,  near  Lima,  Mr.  Darwin  found,  at  the 
altitude  of  eighty-five  feet  above  the  sea,  pieces  of  cotton-thread, 
plaited  rush,  and  the  head  of  a  stalk  of  Indian  corn,  all  of  which 
had  evidently  been  imbedded  with  the  shells.  At  the  same 
height  on  the  neighbouring  mainland,  he  found  other  signs  cor- 
roborating the  opinion  that  the  ancient  bed  of  the  sea  had  there 
also  been  uplifted  eighty-five  feet,  since  the  region  was  first  peo- 
pled by  the  Peruvian  race.*  But  similar  shells,  or  strata,  con- 
taining them,  have  been  found  much  higher,  almost  everywhere 
between  the  Andes  and  the  sea  coasts  of  Chili  and  Peru,  in  which 
no  human  remains  were  ever,  or  in  all  probability  ever  will  be, 
discovered.  These  strata,  therefore,  may  provisionally,  at  least, 
be  designated  Newer  Pliocene. 

In  the  West  Indies,  also,  rocks  both  of  the  Recent  and  Newer 
Pliocene  periods  abound.  Thus,  a  solid  limestone  occurs  at  the 
level  of  the  sea-beach  in  the  island  of  Guadaloupe,  enveloping 

*  Journal,  p.  451. 


172      LYELL'S  ELEMENTS  OF  GEOLOGY. 


Recent  and  Newer  Pliocene  Strata. 


human  skeletons.  The  stone  is  extremely  hard,  arid  chiefly  com- 
posed of  comminuted  shell  and  coral,  with  here  and  there  some 
entire  corals  and  shells,  of  species  now  living  in  the  adjacent 
sea.  With  them  are  included  arrow-heads,  fragments  of  pottery, 
and  other  fabricated  articles.  A  limestone  with  similar  contents 
has  been  formed,  and  is  still  forming,  in  St.  Domingo  and  other 
islands.  But  there  are  also  more  ancient  rocks  in  the  West 
Indian  Archipelago,  as  in  Cuba,  near  the  Havanna,  and  in  other 
islands  in  which  are  shells  identical  with  those  now  living  in 
corresponding  latitudes ;  some  well  preserved,  others  in  casts,  all 
referable  to  a  period  which,  if  we  can  depend  on  negative  evi- 
dence, was  anterior  to  the  introduction  of  man  into  the  New 
World. 

The  history  of  Holland,  during  the  last  2000  years,  makes 
us  acquainted  with  a  vast  accession  of  Recent  strata,  by  which 
parts  of  the  sea  near  the  mouths  of  the  Rhine  have  been  filled 
up  and  converted  into  dry  land.  But,  if  we  ascend  the  Rhine, 
we  find  throughout  its  course,  from  Cologne  to  the  frontiers  of 
Switzerland,  a  yellow  calcareous  loam,  called  loess  by  the  Ger- 
mans, in  which  are  fossil  shells,  both  freshwater  and  terrestrial, 
of  common  European  species.  The  entire  thickness  of  this  loam, 
amounts  in  some  places  to  200  or  300  feet,  and  it  rises  from  the 
height  of  300  to  1200  feet  above  the  sea.  Bones  of  the  mam- 
moth»or  extinct  elephant,  together  with  those  of  the  horse,  and 
some  other  quadrupeds,  have  been  met  with  in  this  Newer  Plio- 
cene formation,  but  no  remains  or  signs  of  man ;  and  it  can  be 
proved  that  the  physical  geography  of  the  whole  valley  of  the 
Rhine  has  undergone  enormous  changes  since  the  deposition  of 
this  loam. 

No  marine  strata  of  the  Recent  period  have  yet  been  brought 
to  light  in  England  which  rise  to  such  a  height  above  the  level 
of  the  sea  as  the  highest  tides  may  not  once  have  reached. 
Buried  ships  have  been  found  in  the  former  channels  of  the 
Rother  in  Sussex,  of  the  Mersey  in  Kent,  and  Thames  near 
London.  Canoes  and  stone-hatches  have  been  dug  up,  in  almost 
all  parts  of  the  kingdom,  in  peat  and  shell-marl ;  but  there  is  no 
evidence,  as  in  Sweden,  Italy,  Peru,  Chili,  and  other  parts  of  the 
world,  of  the  bed  of  the  sea,  and  the  adjoining  coast,  having 
been  uplifted  bodily  in  modern  times,  so  that  Recent  formations 
have  become  land.  There  are,  however,  in  various  parts  of 
Great  Britain  and  Ireland,  Newer  Pliocene  deposits  of  marine 
origin,  consisting  of  sand  and  clay,  usually  of  small  thickness  ; 
as,  for  example,  in  Cornwall,  and  near  the  borders  of  the  great 
estuaries  of  the  Clyde  and  Forth,  in  Scotland,  and  ;n  that  of 
the  Shannon,  in  Ireland.  These  are  found  usually  near  the 


PART  II.     CHAPTER  XIV.  173 


Recent  and  Newer  Pliocene  Strata. 


coast,  but  in  some  rare  instances  they  penetrate  inland  to  a  dis- 
tance of  sixty  miles  from  the  sea,  as  at  Bridgnorth,  in  Shrop- 
shire/* They  also  rise  occasionally  to  great  heights,  as  at 
Preston,  in  Lancashire,  where  they  are  350  feet  above  the  sea  ; 
and,  what  is  still  more  remarkable,  on  a  mountain  called  Moel 
Tryfane,  in  Wales,  near  the  Menai  Straits,  they  attain  an  eleva- 
tion of  about  1400  feet.f  In  all  these  places  they  contain  shells 
indisputably  of  the  same  species  as  those  which  now  people  the 
British  seas  ;  and  although,  perhaps,  on  more  accurate  examina- 
tion some  slight  intermixture  of  extinct  testacea  will  appear,  yet 
the  geologist  will  always  refer  them  to  the  most  modern  tertiary 
era. 

There  are,  moreover,  a  great  many  freshwater  deposits  scatter- 
ed over  England,  which  belong  to  the  Newer  Pliocene  period,  as 
at  North  Cliff,  in  the  county  of  York,  where  thirteen  species  of 
British  land  and  freshwater  shells  were  found  imbedded  in  the 
same  strata  with  the  remains  of  the  bison  and  mammoth.:]:  In 
like  manner,  at  Cropthorne,  in  Worcestershire,  on  the  banks  of 
the  Avon,  a  tributary  of  the  Severn,  Mr.  Strickland  observed  flu- 
viatile  and  land  shells,  nearly  all  of  recent  species,  with  the  bones 
of  an  extinct  kind  of  hippopotamus.  Recent  freshwater  shells 
also  appear  in  beds  of  loam,  together  with  bones  of  the  deer 
and  mammoth,  in  the  cliffs  of  the  estuary  of  the  Stour,  in  Suffolk. 
Some  writers  have  confounded  these  and  similar  fluviatile  and 
lacustrine  strata,  with  the  ancient  alluviums  which  they  term 
diluvial. 

Older  Pliocene  strata  in  England — Crag. — There  are  some 
few  countries  in  Europe,  as  in  the  district  between  the  Gironde 
and  the  Pyrenees,  in  the  south  of  France,  or  that  between  the 
Alps,  north  of  Vicenza,  and  the  hills  near  Turin,  in  the  north  of 
Italy,  where  fossiliferous  strata  representing  all  the  three  periods, 
the  Eocene,  Miocene,  and  Older  Pliocene,  are  present.  But  the 
tertiary  deposits  of  England  are  limited  to  the  Eocene  and  the 
Older  and  Newer  Pliocene  groups,  the  Miocene  being  wanting. 

It  is  chiefly  in  the  eastern  part  of  the  county  of  Suffolk  that  a 
deposit  provincially  named  crag  is  seen  in  its  most  characteristic 
form.  This  crag  consists  chiefly  of  a  series  of  thin  layers  of 
quartzose  sand  and  comminuted  shell,  which  rest  sometimes  on 
chalk,  sometimes  on  an  Eocene  tertiary  formation,  called  the 
"  London  Clay."  Mr.  Charlesworth,  whose  opinion  I  have  lately 
had  opportunities  of  confirming,  has  correctly  stated  that  the 
crag,  in  part  of  Suffolk,  may  be  divided  into  two  distinct  masses, 

*  See  Murchison,  Proceedings  of  Geol.  Soc.,  vol.  ii.  p.  333. 
t  Proceedings  of  Geol.  Soc.,  vol.  i.  p.  331.,  and  vol.  ii.  p.  333. 
t  See  Principles  of  Geology,  Index,   "  Mammoth." 

P* 


174      LYELL'S  ELEMENTS  OF  GEOLOGY. 


Older  Pliocene  Strata English  Crag. 

the  upper  of  which  may  be  termed  the  red,  and  the  lower  the 
coralline  crag.*  The  inferior  division,  however,  is  of  very 
limited  extent,  ranging  over  an  area  about  twenty  miles  in 
length,  and  three  or  four  miles  in  breadth,  between  the  rivers 
Aide  and  Stour. 

The  red  crag  is  generally  at  once  distinguishable  from  the 
coralline,  by  the  deep  red  ferruginous  or  ochreous-colour  of  its 
sands  and  fossils.  Its  strata  are  also  remarkable  for  the  oblique 
or  diagonal  position  of  the  subordinate  layers  (see  p.  32.)  ;  and 
these  often  consist  of  small  flat  pieces  of  shell,  which  lie  parallel 
to  the  planes  of  the  smaller  strata,  showing  clearly  that  they 
were  so  deposited,  and  that  this  structure  has  not  been  due  to  any 
subsequent  rearrangement  of  the  mass  after  deposition.  That 
the  ancient  sandbanks  in  question  had  sometimes  sides  sloping  in 
all  directions,  is  implied  by  the  fact  that  the  oblique  layers  some- 
times slant  towards  all  points  of  the  compass  in  different  parts 
of  the  same  quarry.  They  were  probably  shifting  sands,  and  a 
great  proportion  of  the  shells  composing  them  have  been  ground 
down  to  small  pieces,  while  others  have  been  rolled ;  and  the 
two  parts  of  the  bivalves  are  almost  invaricfbly  disunited.  The 
red  crag  contains  some  peculiar  fossils,  and  others  which  seem  to 
have  been  washed  out  of  the  lower  or  coralline  crag.  Some 
few  of  the  bivalves  of  the  red  crag  are  entire,  with  both  valves 
joined. 

The  coralline  crag  is  usually  free  from  ferruginous  stains,  and 
consists  of  light  greenish  shelly  marl,  and  white  calcareous  sand. 
Sometimes  it  forms  a  soft  building  stone,  in  which  entire  shells, 
echini,  and  many  zoophytes  are  imbedded.  Here  and  there  the 
softer  mass  is  divided  by  thin  flags  of  hard  limestone,  in  which 
are  corals  in  a  good  state  of  preservation,  which  evidently  grew 
at  the  bottom  of  a  tranquil  sea,  in  the  position  in  which  we  now 
see  them.  Yet  the  sands  in  this  formation,  as  in  the  red  crag, 
are  often  composed  entirely  of  comminuted  shell. 

In  some  places,  as  at  Tattingstone,  in  Suffolk,  the  lithological 
distinction  of  the  two  divisions,  though  perceptible,  is  much  less 
marked ;  the  inferior  crag  being  composed  chiefly  of  greenish 
marl,  with  only  a  few  stony  beds.  The  shells  also  are  mostly 
broken,  and  corals  are  almost  as  rare  as  in  the  red  crag. 

At  some  places,  as  near  Orford,  the  coralline  crag  is  exposed 
at  the  surface,  and  the  bottom  of  it  has  not  been  reached  at  the 
depth  of  fifty  feet.  Yet  not  far  from  this  town,  the  surface  is 
occupied  exclusively  with  red  crag,  which  rests  immediately  upon 
the  London  clay.  Wherever  the  two  divisions  are  found  toge- 
ther, the  coralline  mass  is  the  lower  of  the  two,  and  is  interposed 

*  London  and  Edin.  Phil.  Mag.  No.  38.  p.  81.  Aug.  1836. 


PART  II.     CHAPTER  XIV. 


175 


Older  Pliocene  Strata English  Crag. 


between  the  red  crag  and  London  clay  ;  and  the  strata  of  the 
upper  and  lower  crag  are  unconformable  one  to  the  other,  as  in 
the  section  represented  in  the  annexed  diagram,  which  I  have 
myself  examined. 

Fig.  128. 


a.  red  craj 


Section  near  Ipswich,  in  Suffolk, 
b.  coralline  crag. 


c.  London  clay. 


In  the  places  here  referred  to,  the  coralline  crag  varies  in 
thickness  from  fifteen  to  more  than  twenty  feet,  and  the  red  crag 
is  often  much  thicker. 

Amongst  upwards  of  400  species  of  testacea  found  in  the 
crag,  there  are  many  common  to  both  divisions ;  but  some, 
which  are  very  abundant  in  the  red,  have  never  been  met  with 
in  the  coralline  crag ;  as,  for  example,  the  Fusus  contrarius 
(Fig.  129.),  and  several  species  of  Nassa  and  Murex,  (see  Figs. 


Fig.  129. 


Fossils  characteristic  of  the  Red  Crag. 


Fig.  130. 


Fig.  131. 


Nassa  granulata. 

Fig.  132. 


Fusus  contrarius. 


Murex  alveolatus. 


Cyprcea  coccinelloides. 


130,  131.),  which  two  genera  seem  never  to  have  been  disco- 
vered in  the  lower  crag.  On  the  other  hand,  scarcely  any  corals 
have  been  found  in  the  red  crag.  These  abound  in  the  inferior 
division,  and  some  of  them  are  of  a  globular  form,  and  belong 
to  the  genera  Theonoa  (Lamoroux),  Cellepora,  and  a  third  to 
Fascicularia,  which  is  of  very  peculiar  structure,  unknown  in 
the  living  creation.  (See  Fig.  133.) 


176 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Older  Pliocene  Strata English  Crag. 


Fig.  133. 


Fascicularia  auruntium,  Milne  Edwards.    Family,   Tubuliporidce,  of  same  author- 
A  coral  of  an  extinct  genus,  from  the  inferior  or  coralline  crag,  Suffolk. 

a.  exterior.  b.  vertical  section  of  interior. 

e.  portion  of  exterior  magnified. 

d.  portion  of  interior  magnified,  showing  that  it  is  made  up  of  long,  thin, 
straight  tubes,  united  in  conical  bundles. 

The  general  analogy  of  the  crag  shells  to  those  now  living  in 
the  neighbouring  seas,  between  the  latitudes  50°  and  60°  north, 
is  so  striking  that  we  cannot  hesitate  to  refer  the  formation  to  the 
Pliocene  period ;  but,  as  all  conchologists  are  agreed  that  more 
than  half  the  species  are  extinct  or  unknown,  it  is  to  the  Older 
and  not  the  Newer  Pliocene  period  that  they  belong.  Dr.  Beck, 
after  examining  260  species  of  shells,  informs  me  that  the  average 
number  of  species  to  genera  is  such  as  indicates  a  temperate  cli- 
mate, a  result  which  is  also  confirmed  by  the  large  development  of 
certain  northern  forms,  such  as  the  genus  Astarte  (see  Fig.  134.), 

Fig.  134. 


dstarte,  (Crassina,  Lam.);  species  common  to  upper  and  lower  crag: 

of  which  there  are  about  fourteen  species,  many  of  them  being 
rich  in  individuals  ;  and  there  is  an  absence  of  genera  peculiar 
to  hot  climates,  such  as  Conus,  Oliva,  Mitra,  Fasciolaria,  and 
others.  The  cowries  (Cypraa)  (Fig.  132.)  also  are  small, 


PART  II.     CHAPTER  XIV.  177 

Older  Pliocene  Strata English  Crag. 

as  in  the  colder  regions.     A  large  volute,  called  Valuta  Lam- 
berti,  (Fig.  135.)  may  seem  an  exception ;  but  it  differs  in  form 
from  the  volutes  of  the  torrid  zone,  and  may,  like 
Fig.  135.     the  large   Voluta  Magellanica,  have  been  extra- 
tropical. 

When  I  first  submitted  the  shells  of  the  crag  to 
M.  Deshayes,  in  1829,  he  recognized  their  general 
resemblance  to  the  fauna  of  the  German  ocean,  and 
determined  that  out  of  1 1 1  species  there  were  45 
identical  with  those  now  living.  Dr.  Beck,  on  the 
other  hand,  who  has  since  seen  much  larger  collec- 
tions, considers  that  almost  all  the  species  are  dis- 
tinguishable from  those  now  living,  and  this  subject 
is  still  under  discussion. 

Lambent,  It  has  been  asked  whether,  as  the  upper  and 
ld'  lower  crag  of  Suffolk  differs  greatly  in  mineral 
composition  and  fossils,  they  may  not  belong  to  two  different  ter- 
tiary periods.  To  this  I  may  reply,  that  the  general  character 
of  the  shells  is  the  same,  and  by  no  means  leads  to  such  a  con- 
clusion. The  two  deposits  may  have  been  going  on  contempo- 
raneously under  different  geographical  conditions  in  the  same 
sea.  One  region  of  deep  and  clear  water,  far  from  the  shore, 
may  have  been  fitted  for  the  growth  of  certain  corals,  echini, 
and  testacea  ;  while,  another  shallower  part  nearer  the  shore,  and 
more  frequently  turbid,  or  where  sand  and  shingle  were  occa- 
sionally drifted  along,  may  have  been  favourable  to  other  species. 
After  this,  the  region  of  deep  and  tranquil  water  becoming  shal- 
low, or  exposed  to  the  action  of  waves  and  currents,  a  formation 
like  the  coralline  crag  may  have  been  covered  over  with  sandy 
deposits,  such  as  the  red  crag,  and  many  fossils  of  the  older  beds 
may  have  been  washed  into  the  newer  strata.  If  a  considerable 
lapse  of  time  intervened  in  a  particular  spot  between  the  conver- 
sion of  a  deep  sea  into  a  shoal,  some  small  change  in  organic 
life  may  have  taken  place,  and  consequently  the  distinctness  in 
character  of  the  fossils  of  the  two  formations  may  be  derived 
from  two  causes,  first  and  principally  the  difference  of  geo- 
graphical conditions,  and,  secondly,  that  law  of  the  coming  in 
and  going  out  of  species  which  was  alluded  to  in  the  last  chap- 
ter, (p.  161.) 

The  area  over  which  both  divisions  of  the  crag  can  be  traced 
is  too  small  to  enable  us  to  arrive  at  satisfactory  conclusions  on 
a  question  of  such  magnitude ;  but  the  section  given  above  (p. 
175.)  shows  distinctly  that,  near  Sutton,  the  lower  crag  had  suf- 
fered much  denudation  before  the  deposition  of  the  red  crag.  At 
D  (Fig.  128.)  there  is  not  only  a  distinct  cliff,  eight  or  ten  feet 


178  LYELL'S  ELEMENTS  OF  GEOLOGY. 


Eocene  Strata London  Clay. 


high,  of  coralline  crag,  running  in  a  direction  N.  E.  and  S.  W., 
against  which  the  red  crag  abuts  with  its  horizontal  layers,  but 
this  cliff  occasionally  overhangs.  The  rock  composing  it  is 
drilled  everywhere  by  Pholades  belonging  to  the  period  of  the 
red  crag.  The  cliff  may  have  been  caused  by  submarine  denu- 
dation, in  a  shallow  sea ;  and  had  the  red  crag  been  equally 
solid,  it  would  probably  have  presented  many  similar  perpendicu- 
lar cliffs ;  for  beds,  ten  or  twelve  feet  thick,  of  loam  or  sand,  in 
this  formation,  are  often  seen  to  be  unconformable  to  older  beds, 
which  have  been  in  part  cut  away.  Similar  excavations  are  now 
made,  even  on  a  larger  scale,  by  the  sea,  in  the  great  sandbanks 
off  Yarmouth,  in  part  of  which  Captain  Hewett,  R.  N.,  found,  in 
1836,  a  broad  channel,  sixty-five  feet  deep,  where  there  had  been 
only  a  depth  of  four  feet  in  1822.  This  remarkable  change  was 
ascertained  during  two  hydrographical  surveys,  in  the  years 
above  mentioned,  and  shows  how  denudation,  amounting  to  sixty 
feet  in  vertical  depth-,  can  take  place  under  water  in  the  course 
of  fourteen  years.  The  new  channel  thus  formed,  serves  now 
(1838)  for  the  entrance.of  ships  into  Yarmouth  Roads. 

Eocene  formations  in  England — London  Clay. — In  the  sec- 
tion already  given  of  the  tertiary  strata  of  Suffolk  (p.  175.),  it 
will  be  seen  that  the  crag  rests  on  a  formation  called  the  London 
clay,  which  there  consists  of  alternating  beds  of  blue  and  brown 
clay,  with  many  nodules  of  calcareous  stone,  used  for  Roman 
cement.  This  formation  is  well  seen  in  the  neighbouring  cliffs 
of  Harwich,  where  the  nodules  contain  many  marine  shells,  and 
sometimes  the  bones  of  turtles.  The  relative  position  of  the 
chalk,  London  clay,  and  crag,  between  the  coast  of  Essex  and 
the  interior,  may  be  understood  by  reference  to  the  annexed  dia- 

Fig.  136. 

Crag.  London  clay.  Chalk. 


gram.  The  London  clay  has  been  so  named,  because  it  occurs 
in  the  neighbourhood  of  the  metropolis,  in  a  trough  or  basin  of 
the  chalk.  (See  section,  p.  182.)  We  know,  by  numerous 
borings  made  for  water,  that  the  chalk  exists  everywhere  below, 
after  we  have  penetrated  through  clay  and  sand  to  the  depth  of 
from  200  to  600  feet ;  and,  if  we  proceed  to  the  south  of  Lon- 
don, we  find  the  chalk  rising  up  to  the  surface  and  forming  the 
Surrey  hills ;  while  if  we  proceed  northwards,  into  Hertford- 
shire, or,  westward,  by  the  Thames,  into  Oxfordshire,  we  again 
meet  with  the  same  chalk. 


PART  II.     CHAPTER  XIV. 


179 


Eocene  Strata Fossil  Shells. 


The  overlying  Eocene  deposit  consists  of  two  portions ;  the 
upper  of  blue  clay,  with  occasional  cement  stones,  as  before 
mentioned  ;  the  lower  of  various  coloured  sands  and  clays  ;  the 
fossils  throughout  all  the  beds  being  very  different  from  those  of 
the  crag.  Scarcely  any  one  of  the  shells  can  be  identified  with 
species  now  living;  and  the  whole  assemblage  is  such  as  to 
resemble  the  testaceous  fauna  of  the  tropics.  This  opinion  is 
favoured  by  the  occurrence  of  many  species  of  Mitra  and  Voluta, 
a  large  Cyprae,  a  very  large  Rostellaria,  and  shells  of  the  genera 

FOSSIL  SHELLS  OF  THE  LONDON  CLAY. 
Fig.  137.  Fig.  138. 


Mitra  scalra. 


Fig.  139. 


Orassatella  sulcata. 


Rostellaria  macroptera,  Sow. 
one  third  of  nal.  size. 


Fig.  140. 


Fig.  141. 


Fig.  142. 


Nautilus  cenlralis. 


Voluta  athleta. 


180      LYELL'S  ELEMENTS  OF  GEOLOGY. 


Fossil  Mammalia. 


Terebellum,  Cancellaria,  Crassatella,  and  others,  with  four  or 
more  species  of  Nautilus.  (See  Figures,  p.  179.)  There  are 
fish,  also,  which  indicate  a  warm  climate ;  among  which  may  be 
mentioned  a  sword-fish,  (Tetrapterus  prisons,  Agassiz,)  about 
eight  feet  long,  and  a  saw-fish,  (Pristis  bisvlcatus,  Ag.)  about 
ten  feet  in  length ;  genera  foreign  to  the  British  seas. 

These  last  have  been  found  in  the  island  of  Sheppey,  which 
is  composed  of  London  clay,  where  also,  as  I  learn  from  M. 
Agassiz,  the  remains  of  no  less  than  fifty  other  species  of  fish 
have  been  discovered.  It  does  not  appear  that  the  fossil  plants 
and  fruits,  so  numerous  in  this  same  island,  or  the  fossil  plants 
of  the  corresponding  Eocene  formation  of  Paris,  have  by  any 
means  so  tropical  an  aspect  as  the  shells,  but  rather  indicate  such 
a  flora  as  might  be  found  on  the  borders  of  the  Mediterranean. 

Besides  the  marine  formation  called  London  clay,  there  are 
freshwater  strata  of  the  Eocene  period  in  the  Isle  of  Wight,  and 
opposite  coast  of  Hampshire.  They  contain  shells,  such  as 
Limnea,  and  Planorbis  ;  Gyrogonites,  or  the  fossil  seeds  of 
Chara  (see  p.  47.)  ;  and  the  bones  of  several  quadrupeds  of  ex- 
tinct genera,  such  as  Palasotherium,  Anoplotherium,  and  Choero- 
potamus,  which  were  lately  found  by  the  Rev.  W.  D.  Fox,  near 
Binstead. 

It  has  already  been  remarked  that  fossil  mammalia  of  extinct 
species  have  been  met  with  in  the  Newer  Pliocene  deposits  of 
England  and  other  countries.  Different  species  characterize  the 
Miocene,  and  others  are  proper  to  the  Eocene  formations ;  and 
among  them  nearly  every  order  and  family  of  the  herbivorous 
and  carnivorous  tribes  are  represented ;  but  those  which  inhabit 
trees  are  most  rare ;  and  it  was  not  until  very  lately,  namely  in 
1837,  that  any  remains  of  quadrumana,  or  of  the  ape  and  mon- 
key tribe,  were  discovered.  These  were  obtained  about  the  same 
time  in  France  and  India ;  in  France,  by  M.  Lartet,  near  Auch, 
in  the  department  of  Gers,  about  forty  miles  west  of  Toulouse, 
where  the  bones  of  an  ape,  or  gibbon,  accompanied  those  of  the 
rhinoceros,  dinotherium,  mastodon,  and  others;  in  India,  by 
Captain  Cautley  and  Dr.  Falconer,  who  found  the  remains  of  a 
monkey,  with  the  bones  of  many  extinct  quadrupeds,  in  the 
Sewalik  hills,  a  lower  range  of  the  Himalaya  mountains,  near 
Saharuripore. 

*  The  frequent  occurrence  in  the  tertiary  strata  of  fossils  refera- 
ble to  the  highest  class  of  vertebrata  is  a  fact  the  more  worthy 
of  notice,  as  we  shall  find  in  the  sequel  how  great  is  their  rarity 
in  the  secondary  formations. 


PART  II.     CHAPTER  XV. 


Cretaceous  Group. 


CHAPTER  XV. 


CRETACEOUS    GROUP. 

White  chalk — Its  marine  origin  shown  by  fossil  shells — Extinct  genera  of 
cephalopoda — Sponges  and  corals  in  the  chalk— No  terrestrial  or  fluviatile  shells, 
no  land  plants  —  Supposed  origin  of  white  chalk  from  decomposed  corals  — 
Single  pebbles,  whence  derived  —  Cretaceous  coral-reef  in  Denmark  —  Maes- 
tricht  beds  and  fossils  —  Origin  of  flint  in  chalk  —  Wide  area  covered  by  chalk 
—  Green-sand  formation  and  fossils — Origin  of — External  configuration  of 
chalk  —  Outstanding  columns  or  needles —  Period  of  emergence  from  the  sea  — 
Difference  of  the  chalk  of  the  north  and  south  of  Europe — Hippurites  —  Num- 
mulites  —  Altered  lithological  character  of  cretaceous  formation  in  Spain  and 
Greece — Terminology. 

THE  next  group  which  succeeds  to  the  tertiary  strata  in  the 
descending  order  has  been  called  Cretaceous  or  chalky,  because 
it  consists  in  part  of  that  remarkable  white  earthy  limestone  called 
chalk  (creta).  With  this  limestone  however  are  usually  asso- 
ciated other  deposits  of  sand,  marl,  and  clay,  called  the  Green- 
sand  formation,  because  some  of  its  sands  are  remarkable  for 
their  bright  green  colour. 

The  following  are  the  subdivisions  into  which  the  Cretaceous 
Strata  have  been  divided  in  the  south  of  England : — 

a.  soft  white  chalk,  with 
flints 

b.  hard  white  chalk,  with 
few  or  no  flints 

c.  chalk  marl 

a.  upper  green-sand    - 

b.  Gault,  or  blue  marl,  10  to 

c.  lower  green-sand  and    } 
iron-sand,    with  occa-    >    250.t 
sional  limestone       -       ) 

The  accompanying  section  (Fig.  143.)  will  show  the  manner 
in  which  the  tertiary  strata  of  the  London  and  Paris  basins,  as 
they  are  called,  rest  upon  the  chalk,  and  how  the  white  chalk  in 
its  turn  reposes  throughout  this  region  upon  the  green-sand  form- 
ation. 


1.  Chalk 

formation.      1 

Cretaceous 

group. 

2.  Green-sand 

formation. 

*  Conybeare,  Outline,  &c.,  p.  85. 

t  Fitton,  Geol.  Trans.,  Second  Series,  vol.  iv.  p.  319. 


182 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Fossils  of  the  Chalk. 


I  shall  now  speak  first  of  the  chalk, 
its  fossils,  and  probable  origin  ;  and  then 
say  something  of  the  green-sand ;  after 
which  I  shall  pou*t  out  the  probable  rela- 
tions of  the  chalk  and  green-sand  to  each 
other. 

White  Chalk. — The  white  chalk  used 
in  writing  consists  almost  purely  of  car- 
bonate of  lime.     Although  usually  soft, 
this  substance  passes  in  some  districts  by 
a  gradual  change  into  a  solid  stone  used 
for  building.     The  stratification  is  often 
|       obscure,  except  where  rendered  distinct 
by   alternating   layers   of  flint.     These 
«       layers  are  from  two  to  four  feet  distant 
g       from  each  other,  and  from  three  to  six 
inches  in  thickness,  occasionally  in  con- 
J      tinuous   beds,   but    more   frequently    in 
nodules. 

The  annexed  figures   represent  some 
s       few  of  the  fossil  shells  which  are  abun 
dant  in  the  white  chalk,  and  these  alone 
|       are  sufficient  to  prove  its  marine  origin. 
1       Some  of  them,  such  as  the  Terebratula3, 
I4      (see   Figs.    148.   150,    151,    152.)   are 
jj       known  to  live  at  the  bottom  of  the  sea, 
£       where  the  water  is  tranquil,  and  of  some 
§       depth.     The  Crania  and  Catillus  (Figs. 
145.  and  144.)  may  be  pointed  out  as 
forms  which,  so  far  as  our  present  infor- 
mation extends,  became  extinct   at   the 
close  of  the  cretaceous  period,  and  are 
therefore  never  met  with  in  any  tertiary 
stratum,  or  in   a  living  state.     Among 
other  forms,  equally  conspicuous  among 
the    fossil    mollusca   of    the   cretaceous 
group,  and  foreign  to  the  tertiary  and 
recent    periods,  may  be   mentioned   the 
Belemnite,  Ammonite,  Baculite,  and  Tur- 
rilite  of  the  family  Cephalopoda,  to  which 
the  living  cuttle-fish  and  nautilus  belong. 
One  of  these,  the  Belemnite,  like  the  bone  of  the  common 
cuttle-fish,  was  an  internal  shell.     Besides  these  there  are  other 
fossils  in  the  chalk,  such  as  sea-urchins,  corals,  and  sponges, 


PART  II.     CHAPTER  XV. 


183 


Fossils  of  the  White  Chalk. 


FOSSILS  OF  THE  WHITE  CHALK. 
Fig.  144. 


Catillus  Cuvicri.    (Syn.  Inoceramus  Cuvieri,  Sow.) 

Fig.  147. 


Fig.  145. 


Crania  Parisiensis, 
inferior  or  attached 
valve. 


Fig.  148. 


Fig.  150. 


Fig.  146. 


Plagiostoma 
Hoperi. 


Plagiostoma  spinosum. 


Fig.  149. 


Ostrea  carinata, 
also  in  Upper  Green-sand. 


Fig.  151. 


Fig.  152. 


Terebratula 

octoplicata, 

(Var.  of  T.  plicatilis.) 


Torebratula  pumilus. 
(Magas  pumilua,  Sow.) 


184 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Fossil  Cephalopoda  of  Extinct  Genera. 


Fig.  153. 


Ostrea  vesicularis  (Gryphaa  globosa,  Sow.)' 
also  in  Upper  Green-sand. 


FOSSIL  CEPHALOPODA  OF  EXTINCT  GENERA. 

Cretaceous  Period. 

Fig.  154. 


a.  Turrilites  costatus,  Gault. 

b.  Same,  showing  the  indented  border  of  the  partition  of  the  chambtra. 


Fig.  155. 


^.e  Chalk  and  Upper  Greenland. 


Pig.  166. 


Portion  of  Baculites  Faujasii, 
White  Chalk  and  Upper  Green -sand. 


Fig.  157. 


Portion  of  Baculites  anceps, 
White  Chalk  and  Upper  Green-sand. 


PART  II.     CHAPTER  XV. 


185 


Fossils  of  the  Chalk. 


(see  Figures,)  which  are  alike  marine.     They  are  dispersed 
indifferently  through  the  soft  chalk  and  the  hard  flint. 


Fig.  158. 


rfnanchytes  ovatus. 

a.  side  view. 

b.  bottom  of  the  shell  on  which  both  the  oral  and  anal  apertures  are  placed  ; 

the  anal  being  to  the  right,  and  more  oval. 


Fig.  159. 


Eschara  disticha. 
a.  natural  size.  b.  portion  magnified. 

To  some  of  these  inclosed  zoophytes  many  flints  owe  their 
irregular  forms,  as  in  the  flint  represented  in  Fig.  161.,  where 
the  hollows  on  the  exterior  are  caused  by  the  branches  of  a 
sponge,  which  is  seen  on  breaking  open  the  flint.  (See  Fig,  160.) 


Fig.  160. 


Fig.  161. 


A  branching  sponge  in  a  flint  from  the  chalk. 


186  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Origin  of  White  Chalk.  ** 

With  these  fossils  the  remains  of  fish  and  Crustacea  are  not 
uncommon ;  but  we  meet  with  no  bones  of  land  animals,  nor 
any  terrestrial  or  fluviatile  shells,  nor  any  plants,  except  pieces 
of  drift-wood  and  sea-weed,  nor  any  sand  or  pebbles ;  all  the 
appearances  concur  in  leading  us  to  believe  that  this  deposit  was 
formed  in  a  deep  sea,  far  from  land,  and  at  a  time  when  the 
European  fauna  was  perfectly  distinct  from  that  of  the  tertiary 
period,  from  which  its  numerous  species  of  plants  and  animals 
entirely  differ. 

Origin  of  the  White  Chalk. — Having  then  come  to  the  con- 
clusion, that  the  chalk  was  formed  in  an  open  sea  of  some  depth, 
we  may  next  inquire,  in  what  manner  so  large  a  quantity  of  this 
peculiar  white  substance  could  have  accumulated  over  an  area 
many  hundred  miles  in  diameter,  and  some  of  the  extreme 
points  of  which  are  distant,  as  we  shall  see  in  the  sequel,  more 
than  1000  geographical  miles  from  each  other. 

It  was  remarked  in  an  early  part  of  this  volume,  that  some 
even  of  that  chalk  which  appears  to  an  ordinary  observer  quite 
destitute  of  organic  remains,  is  nevertheless  seen  under  the 
microscope  to  be  full  of  fragments  of  corals  and  sponges  ;  the 
valves  of  Cytherina,  the  shells  of  foraminifera,  and  still  more 
minute  infusoria.  (See  p.  41.) 

Now  it  had  been  often  suspected  before  these  discoveries,  that 
white  chalk  might  be  of  animal  origin,  even  where  every  trace 
of  organic  structure  has  vanished.  This  bold  idea  was  partly 
founded  on  the  fact,  that  the  chalk  consisted  of  pure  carbonate 
of  lime,  such  as  would  result  from  the  decomposition  of  testacea, 
echini,  and  corals,  and  in  the  passage  observable  between  these 
fossils  when  half  decomposed  into  chalk.  But  this  conjecture 
seemed  to  many  naturalists  quite  vague  and  visionary,  until  its 
probability  was  strengthened  by  new  evidence  -brought  to  light 
by  modern  geologists. 

We  learn  from  Lieutenant  Nelson,  that,  in  the  Bermuda 
islands,  there  are  several  basins  or  lagoons  almost  surrounded 
and  inclosed  by  reefs  of  coral.  At  the  bottom  of  these  lagoons 
a  soft  white  calcareous  mud  is  formed  by  the  decomposition  of 
Eschara,  Flustra,  Cellepora,  and  other  soft  corallines.  This 
mud,  when  driedj  is  undistinguishable  from  common  white  earthy 
chalk ;  and  some  portions  of  it,  presented  to  the  Museum  of  the 
Geological  Society  of  London,  might,  after  full  examination,  be 
mistaken  for  ancient  chalk,  but  for  the  labels  attached  to  them. 
About  the  same  time  Mr.  C.  Darwin  observed  similar  facts  in  the 
coral  islands  of  the  Pacific ;  and  came  also  to  the  opinion,  that 
much  of  the  soft  white  mud  found  at  the  bottom  of  the  sea  near 
coral  reefs  has  passed  through  the  bodies  of  worms,  by  which 


JPART  II.    CHAPTER  XV.  187 

Pebbles  in  Chalk,  whence  derived. 

the  stony  masses  of  coral  are  everywhere  bored ;  and  other  por- 
tions through  the  intestines  of  fish ;  for  certain  gregarious  fish 
of  the  genus  Spams  are  visible  through  the  clear  water,  brows- 
ing quietly,  in  great  numbers,  on  living  cor- 
Fig.  162.  Fig.  163.  ais>  \{\^e  grazing  herds  of  graminivorous 
quadrupeds.  On  opening  their  bodies,  Mr. 
Darwin  found  their  intestines  filled  with 
impure  chalk.  This  circumstance  is  the 
more  in  point,  when  we  recollect  how  the 
fossilist  was  formerly  puzzled  by  meeting 
with  certain  bodies,  called  cones  of  the 
larch,  in  chalk,  which  were  afterwards 
offish  called  fuio-  recognized  by  Dr.  Buckland  to  be  the  ex- 
n,  from  the  chalk.  crement  of  fish.*  These  spiral  coprolites 
(see  Figures)  like  the  scales  and  bones  of  fossil  fish  in  the  chalk, 
are  composed  chiefly  of  phosphate  of  lime. 

Single  pebbles  in  chalk.  —  The  general  absence  of  sarj.d 
and  pebbles  in  the  white  chalk  has  been  already  mentioned ; 
but  the  occurrence  here  and  there  of  a  few  isolated  pebbles 
of  quartz  and  green-schist,  some  of  them  two  or  three  inches 
in  diameter,  in  the  south-east  of  England,  has  justly  excited 
much  wonder.  If  these  had  been  carried  to  the  spots  where 
we  now  find  them  by  waves  or  currents  from  the  lands  once 
bordering  the  cretaceous  sea,  how  happened  it  that  no  sand 
or  mud  were  transported  thither  at  the  same  time  ?  We  cannot 
conceive  such  rounded  stones  to  have  been  drifted  like  erratic 
blocks  by  ice,  for  that  would  imply  a  cold  climate  in  the  cre- 
taceous period ;  a  supposition  inconsistent  with  the  luxuriant 
growth  of  large  chambered  univalves,  numerous  corals,  and 
many  fish,  and  other  fossils  of  tropical  forms.  (See  p.  86.) 

Now  in  Keeling  Island,  one  of  those  detached  masses  of  coral 
which  rise  up  in  the  wide  Pacific,  Captain  Ross  found  a  single 
fragment  of  green-stone,  where  every  other  particle  of  matter 
was  calcareous ;  and  Mr.  Darwin  concludes  that  it  must  have 
come  there  entangled  in  the  roots  of  a  large  tree.  He  reminds 
us  that  Chamisso,  a  distinguished  naturalist  who  accompanied 
Kotzebue,  affirms,  that  the  inhabitants  of  the  Radack  archipelago, 
a  group  of  lagoon  islands,  in  the  midst  of  the  Pacific,  obtained 
stones  for  sharpening  their  instruments  by  searching  the  roots 
of  trees  which  are  cast  up  on  the  beach,  f 

It  may  perhaps  be  objected,  that  a  similar  mode  of  transport 
cannot  have  happened  in  the  cretaceous  sea,  because  fossil  wood 

*  Geol.  Trans.  Second  Series,  vol.  iii.  p.  232.  plate  31.  figs.  3.  and  11. 
t  Darwin,  p.  549.    Kotzebue's  First  Voyage,  vol.  iii.  p.  155. 


188  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Cretaceous  Coral  Reef. 

is  very  rare  in  the  chalk.  Nevertheless  wood  is  sometimes  met 
with,  and  in  the  same  parts  of  the  chalk  where  the  pebbles  are 
found,  beth  in  soft  stone  and  in  a  silicified  state  in  flints.  In 
these  cases  it  has  often  every  appearance  of  having  been  floated 
from  a  distance,  being  usually  perforated  by  boring-shells,  such 
as  the  Teredo  and  Fistulana.* 

The  only  other  mode  of  transport  which  suggests  itself  is  sea- 
weed. Dr.  Beck  informs  me,  that  in  the  Lym-Fiord,  in  Jutland, 
the  Fucus  vesiculosus  sometimes  grows  to  the  height  of  ten 
feet,  and  the  branches  rising  from  a  single  root,  form  a  cluster 
several  feet  in  diameter.  When  the  bladders  are  distended,  the 
plant  becomes  so  buoyant  as  to  float  up  loose  stones  several 
inches  in  diameter,  and  these  are  often  thrown  by  the  waves 
high  up  on  the  beach.  The  Fucus  giganteus,  of  Solander,  so 
common  in  Terra  del  Fuego,  is  said  by  Captain  Cook  to  obtain 
the  length  of  360  feet,  although  the  stem  is  not  much  thicker 
than  a  man's  thumb.  It  is  often  met  with  floating  at  sea,  with 
shells  attached,  several  hundred  "miles  from  the  spots  where  it 
grew.  Some  of  these  plants,  says  Mr.  Darwin,  were  found  ad- 
hering to  large  loose  stones  in  the  inland  channels  of  Terra  del 
Fuego,  during  the  voyage  of  the  Beagle  in  1834;  and  that  so 
firmly,  that  the  stones  were  drawn  up  from  the  bottom  into  the 
boat,  although  so  heavy  that  they  could  scarcely  be  lifted  in  by 
one  person.f  Some  fossil  sea-weeds  have  been  found  in  the 
cretaceous  formation,  but  none,  as  yet,  of  large  size. 

Cretaceous  coral  reef  in  Denmark. — Having  said  so  much 
on  the  probable  derivation  of  chalk  from  the  decay  of  corals 
and  shells,  I  may  add,  that  in  the  island  of  Seeland,  in  Denmark, 
there  is  a  yellow  limestone  intimately  connected  with  the  chalk, 
and  containing  a  vast  number  of  the  same  fossils,  which  consists 
of  an  aggregate  of  corals,  retaining  their  forms  as  distinctly  as 
the  dead  zoophytes  which  enter  into  the  structure  of  reefs  now 
growing  in  the  sea.  The  thickness  of  this  rock  is  unknown,  but 
it  has  been  quarried  at  Faxoe  to  the  depth  of  forty  feet.  At 
Stevensklint,  in  Seeland,  it  is  seen  to  rest  on  white  chalk  with 
flints,  from  which  it  differs  greatly  in  appearance,  and  where  it 
is  covered  again  by  another  limestone,  which  although  of  later 
date,  agrees  more  nearly  with  the  white  chalk,  both  in  fossils  and 
mineral  character.  Out  of  104  species  of  sponges,  corals,  and 
other  zoophytes,  collected  from  the  limestone  of  Faxoe,  and  from 
the  ordinary  white  chalk  of  Denmark,  which  agrees  with  that  of 
England,  no  less  than  forty-two  are  common  to  both  formations ; 
and  many  of  the  same  species  of  bivalve  shells  and  echinoder- 

*  Mantell,  Geol.  of  S.  E.  of  England,  p.  96.'  t  Darwin,  p.  303. 


PART  U.     CHAPTER  XV.  189 

Cretaceous  Coral  Reef  ......................  Maastricht  Beds. 

mata  have  been  found  in  both.  The  Faxoe  formation,  however, 
is  not  only  remarkable  for  the  number  and  good  preservation  of 
its  fossil  corals,  but  also  from  the  generic  resemblance  of  many 
of  its  univalve  shells  to  forms  .  usually  supposed  to  appertain 
chiefly  or  exclusively  to  the  tertiary  period.  Thus  among  the 
patelliform  univalves,  we  find  Patella  and  Emarginula,  and  among 
the  spiral,  the  following  genera,  Cyprsea,  Oliva,  Mitra,  Cerithium, 
Fusus,  Trochus,  Triton,  Nassa,  and  Bulla. 

The  species  however  do  not  agree  with  those  of  the  tertiary 
strata,  and  are  associated  with  cephalopoda  of  those  extinct 
families  before  mentioned  as  characteristic  of  the  cretaceous, 
and  foreign  to  the  tertiary  epoch  ;  as,  for  example,  the  ammonite, 
belemnite,  and  baculite.  Two  species,  the  Belemnites  mucronatus 
(Fig.  155.),  and  the  Baculites  Faujasii  (Fig.  156.),  being 
common  to  the  Faxoe  beds  and  white  chalk. 

From  these  facts,  we  may  conclude  that  the  Faxoe  limestone 
was  formed  in  the  cretaceous  sea,  in  a  spot  favourable  for  the 
multiplication  of  stony  corals  and  univalve  shells  ;  and  as  some 
small  portions  of  the  rock  consist  of  white  earthy  chalk,  this 
latter  substance  must  have  been  produced  simultaneously,  and 
some  of  it  may  have  been  washed  away,  in  the  form  of  mud, 
from  the  coral  reef  of  Faxoe,  and  dispersed  over  the  deeper  parts 
of  the  same  ocean,  just  as  the  white  mud,  swept  out  of  the  la- 
goons of  the  Bermudas  or  coral  islets  of  the  Pacific,  must  form 
deposits  of  white  chalk,  covering  much  wider  spaces  than  those 
occupied  by  the  reefs. 

The  same  remarks  apply  to  a  rock,  which  reposes  on  the 
Upper  Chalk  with  flints,  at  St.  Peter's  Mount,  Maestricht,  and  at 
Ciply,  near  Mons.  It  is  a  soft  yellowish  stone,  not  very  unlike 
chalk,  and  "includes  siliceous  masses,  which  are  much  more 
rare  than  those  of  the  chalk,  of  greater  bulk,  and  not  composed 
of  black  flint,  but  of  chert  and  calce- 
dony."*  Like  the  Faxoestone,  it  is 
characterized  by  a  peculiar  assemblage 
of  organic  remains  which  are  specifi- 
cally distinct  from  those  of  the  tertiary 
period,  but  many  of  them  common  to 
the  white  chalk. 

As  these  Maestricht  beds  have  been 
thought  to  be  intermediate  in  character 
Rkotomagevsis,     Between  the  secondary  and  tertiary  for- 


, 
Maestricht  ;  found  by  Count   mations,  it  may  be  proper  to  mention, 

as  opposed  to  this  opinion,  that  the  Am- 
monite (Fig.  164.),  Baculite,  Hamite,  and  Hippurite,  have  been 

*  Fitton,  Geol.  Proceedings,  1830. 


190  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Chalk  Flints. 

found  in  the  Maestricht  limestone,  genera  which  have  not  yet 
been  detected  in  strata  newer  than  the  chalk.  In  the  same 
formation,  also,  large  turtles  have  been  found,  and  a  gigantic 
reptile,  the  Mosasaurus,  or  fossil  Monitor,  some  of  the  vertebrce 
of  which  appear  also  in  the  English  chalk.*  The  osteological 
characters  of  this  oviparous  quadruped  prove  it  to  have  been 
intermediate  between  the  living  Monitors  and  Iguanas  ;  and,  from 
the  size  of  the  head,  vertebras,  and  other  bones,  it  is  supposed 
to  have  been  twenty-four  feet  in  length. 

The  existence  of  such  turtles  and  saurians  seems  to  imply 
some  neighbouring  land,  on  the  sandy  shores  of  which  these 
creatures  may  have  laid  their  eggs.  But  a  few  small  islets  in 
mid  ocean,  like  Ascension,  so  much  frequented  by  turtles,  may 
perhaps  have  afforded  the  required  retreat  to  these  cretaceous 
reptiles. 

Origin  of  the  Jlint  in  chalk. — It  is  difficult  to  give  a  satis- 
factory explanation  of  the  origin  of  the  flint  in  chalk,  whether 
it  occurs  in  nodules  or  continuous  layers.  It  seems  that  there 
was  originally  siliceous  as  well  as  calcareous  earth  in  the  muddy 
bottom  of  the  cretaceous  sea,  at  least  when  the  upper  chalk  was 
deposited.  Whether  both  these  earths  could  have  been  alike 
supplied  by  the  decay  of  organic  bodies  may  be  matter  of  spe- 
culation ;  but  what  was  said  of  the  origin  of  Tripoli  (see  p.  39.) 
shows  how  microscopic  infusoria  can  give  rise  to  dense  masses 
of  pure  flint.  The  skeletons  of  many  living  sponges  consist  of 
needles  or  spicula  of  flint,  and  these  are  found  very  abundantly 
in  the  flints  of  the  chalk.  There  are  also  other  living  zoophytes, 
which  have  the  power  of  secreting  siliceous  matters  from  the 
waters  of  the  sea,  just  as  mollusca  secrete  calcareous  particles. 

From  whatever  source  the  mud  derived  its  silex,  we  may  at- 
tribute the  parallel  disposition  of  the  flinty  layers  to  successive 
deposition.  The  distances  between  the  layers,  says  Dr.  Buck- 
land,  must  have  been  regulated  by  the  intervals  of  precipitation, 
each  new  mass  forming  at  the  bottom  of  the  ocean  a  bed  of 
pulpy  fluid,  which  did  not  penetrate  the  preceding  bed  on  which 
it  rested,  because  the  consolidation  of  this  last  was  so  far  advanc- 
ed as  to  prevent  such  intermixture,  f  Nevertheless  the  separa- 
tion of  the  flint  into  layers,  so  distinct  from  the  chalk,  is  a  sin- 
gular phenomenon,  and  not  yet  accounted  for.  Perhaps,  as  the 
specific  gravity  of  the  siliceous  exceeds  that  of  the  calcareous 
particles,  the  heavier  flint  may  have  sunk  to  the  bottom  of  each 
stratum  of  soft  mud  ? 

*  See  Mantell's  Geol.  of  S.  E.  of  England. 
t  Geol.  Trans.,  First  Series,  vol.  iv.  p.  420. 


PART  II.     CHAPTER  XV.  191 

Area  covered  by  Chalk Green  Sand  Formation. 

Geographical  extent  of  White  Chalk.  —  The  area  over 
which  the  white  chalk  preserves  a  nearly  homogeneous  aspect  is 
so  great  that  geologists  have  often  despaired  of  finding  any  ana- 
logous deposits  of  recent  date ;  for  chalk  is  met  with  in  a  north- 
west and  south-east  direction,  from  the  north  of  Ireland  to  the 
Crimea,  a  distance  of  about  1140  geographical  miles,  and  in  an 
opposite  direction  it  extends  from  the  south  of  Sweden  to  the 
south  of  Bordeaux,  a  distance  of  about  840  geographical  miles. 
But  we  must  not  conclude  that  it  was  ever  spread  out  uniformly 
over  the  whole  of  this  vast  space,  but  merely  that  there  were 
patches  of  it,  of  various  sizes,  throughout  this  area.  '  Now,  if 
we  turn  to  those  regions  of  the  Pacific  over  which  coral  reefs 
are  scattered,  we  find  some  archipelagoes  of  lagoon  islands,  such 
as  that  of  the  Dangerous  archipelago,  for  instance,  and  that  of 
Radack,  with  some  adjoining  groups,  which  are  from  1100  to 
1200  miles  in  length,  and  300  or  400  miles  broad;  and  the 
space  to  which  Flinders  proposed  to  give  the  name  of  the  Coral- 
lian  sea  is  still  larger ;  for  it  is  bounded  on  the  east  by  the  Aus- 
tralian barrier,  on  the  west  by  New  Caledonia,  and  on  the  north 
by  the  reefs  of  Louisiade.  Although  the  islands  in  these  spaces 
may  be  thinly  sown,  the  mud  of  the  decomposing  zoophytes 
may  be  scattered  far  and  wide  by  oceanic  currents. 

Green-sand  formation. — The  lower  division  of  the  Cretace- 
ous group  in  England  is  divisible,  as  we  have  already  seen,  into 
Upper  Green-sand,  Gault,  and  Lower  Green-sand.  The  green 
grains  have  been  found,  by  analysis,  to  consist  chiefly  of  silicate 
of  iron,  and  they  agree  in  composition  with  chlorite.  The  infe- 
rior white  marly  chalk  becomes  more  and  more  charged  with 
these  grains  until  it  passes  into  the  upper  green-sand,  a  forma- 
tion of  sand  and  sandy  marl,  frequently  mixed  with  chert,  and 
this  again  passes  downwards  into  the  clay  and  marl,  provincially 
called  Gault.  Both  of  these  subdivisions,  although  often  dimin- 
ishing in  volume  to  a  thickness  of  two  or  three  yards,  form  dis- 
tinct and  continuous  bands  of  sand  and  clay  between  the  chalk, 
and  lower  green-sand  throughout  considerable  tracts  in  England, 
France,  and  Belgium  ;  and  each  preserves  throughout  this  space 
certain  mineral  peculiarities  and  characteristic  fossils. 

The  lower  green-sand  below  the  gault  is  formed  partly  of 
green  and  partly  of  ferruginous  sand  and  sandstone,  with  some 
limestone.  These  rocks  succeed  each  other  in  the  following 
descending  order  in  Kent : — 

No.  1.  Sand,  white,  yellowish,  or  ferruginous,  with  concretions  of  limestone  and 
chert, ! 70  feet. 

2.  Sand  with  green  matter, 70  to  100  feet. 

3.  Calcareous  stone,  called  Kentish  rag 60  to  80  feet  * 

*  Fitton,  Geol.  Trans.,  Second  Series,  vol.  iv.  p.  319. 


192 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Origin  of  the  Green-sand 'Formation. 


FOSSILS  OF  THE  GREEN-SAND  FORMATION. 

Fig.  166. 
Fig.  165. 


a.  Terebratula  lyra.          Upper  Green-sand. 

b.  Same,  seen  in  profile.        France. 


Fig.  167. 


Pecten  5  costatus. 

Upper  and  lower 

green-sand. 


Hamites  spiniger  (Fitton,)  near  Folkstone.* 


The  fossils  of  the  green-sand  are  marine,  and  some  of  them, 
like  the  Pecten  quinquecostatus,  (Fig.  166.)  range  through  all 
the  members  of  the  series.  Several  forms  of  cephalopoda,  such 
as  the  Hamite,  (Fig.  167.)  Scaphite,  and  others  distinguish  the 
Green-sand  formation  in  England  from  the  White  Chalk. 

Origin  of  the  Green-sand  formation. — Unlike  the  white 
chalk,  this  deposit  consists  of  a  succession  of  ordinary  beds  of 
sand,  clay,  marl,  and  impure  limestone,  the  materials  of  which 
might  result  from  the  wearing  down  of  pre-existing  rocks.  The 
nature  of  these  derivative  rocks  we  learn,  from  finding  in  the 
green-sand  pebbles  of  quartz,  quartzose  sandstone,  jasper,  and 
flinty  slate,  together  with  grains  of  chlorite  and  mica."f"  But  we 
naturally  inquire,  how  it  could  happen  that,  throughout  a  large 
submarine  area,  there  should  be  formed,  first,  a  set  of  mechani- 
cal strata,  such  as  the  green-sand,  and  then  over  the  same  space 
a  pure  zoophytic  and  shelly  limestone,  such  as  the  white  chalk. 
Certain  causes,  which  during  the  first  period  gave  rise  to  depo- 


*  .Fitton,  Geol.  Trans;,  Second  Series,  vol.  iv.  pi.  12. 


t  Ibid.  p.  116. 


PART  II.     CHAPTER  XV.  193 


Green-sand  Formation External  Configuration  of  Chalk. 


sits  of  mud,  sand,  and  pebbles,  must  subsequently  have  ceased 
to  act ;  for  it  is  evident  that  no  similar  sediment  disturbed  the 
clear  waters  of  the  sea  in  which  the  white  chalk  accumulated, 
The  only  hypothesis  which  seems  capable  of  explaining  such 
changes  is  the  gradual  submergence  of  land  which  had  been  pre- 
viously exposed  to  aqueous  denudation.  This  operation  may 
have  gone  on  with  such  slowness  as  to  allow  time  for  consider- 
able fluctuations  in  the  state  of  the  organic  world,  so  that  differ- 
ent sets  of  strata,  beginning  with  the  lower  green-sand,  and  end- 
ing with  the  upper  white  chalk,  may  each  contain  some  peculiar 
remains  of  animals  which  lived  successively  in  the  sea ;  while 
some  species  may  have  continued  to  exist  throughout,  the  whole 
period,  arid  are  therefore  common  to  all  these  formations. 

It  will  be  seen  in  the  next  chapter,  when  we  treat  of  the  strata 
called  the  Wealden,  that  such  a  general  subsidence  of  land  as  is 
here  supposed  to  explain  the  manner  in  which  the  chalk  succeeds 
the  green-sand,  may  be  inferred  from  other  independent  proofs 
to  have  taken  place  throughout  large  areas. 

It  cannot  however  be  assumed,  that  all  the  green-sand  in 
Europe  had  ceased  to  be  deposited  before  any  chalk  began  to 
accumulate.  Such  indeed  was  the  order  of  events  in  parts  of 
England,  France,  Belgium,  and  Denmark ;  but  if  we  compare 
different  countries,  and  some  of  these  not  far  distant  from  each 
other,  we  find  reason  to  believe  that  sand  and  clay  continued  to 
be  thrown  down  in  one  place,  while  pure  chalk  was  forming  in 
another.  In  Westphalia,  for  example,  strata  containing  the  same 
fossils  as  the  white  chalk  of  England,  consist  of  sand  and  marl 
with  green  grains  like  the  upper  green-sand.  Similar  facts  have 
been  observed  in  Hungary  in  the  Carpathian  mountain  chain. 
Such  variations  would  occur  if  the  supposed  sinking  down  of 
land  did  not  take  place  simultaneously  everywhere ;  and  for  this 
reason  the  minor  subdivisions  of  the  cretaceous  group,  however 
persistent  and  uniform  in  their  mineral  characters  in  some 
regions,  vary  rapidly,  and  change  entirely  in  other  directions. 

External  configuration  of  Chalk. — The  smooth  rounded  out- 
line of  the  hills  composed  of  white  chalk  is  well  known  to  all 
who  have  travelled  in  the  south-east  of  England.  The  chalk 
downs,  being  free  from  trees  or  hedge-rows,  afford  us  an  oppor- 
tunity of  observing  the  manner  in  which  the  upper  valleys  unite 
with  larger  ones,  and  how  these  become  wider  and  deeper  as 
they  descend.  For  the  most  part  they  are  dry,  yet  occasionally 
they  afford  a  perfect  system  of  drainage,  when  a  sudden  flood  is 
caused  by  heavy  rains  or  the  melting  of  snow.  We  may  con- 
ceive their  excavation  to  have  been  caused  by  the  action  of  the 
waves  and  currents  while  the  chalk  was  gradually  emerging  from 


194 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Chalk  Needles. 


the  sea.  To  the  same  action  we  may  ascribe  the  escarpments, 
as  they  are  called,  or  those  long  lines  of  precipitous  cliffs  in 
which  the  chalk  often  terminates  abruptly,  and  which,  though 
now  inland,  have  been  undermined  by  the  waves  when  the  chalk 
was  upheaved  from  the  sea. 

Many  examples  occur  in  England  ;  but  there  are  no  precipices 
of  chalk  more  striking  than  those  which  bound  the  lower  part 
of  the  great  valley  or  gorge  through  which  the  Seine  flows  in 
Normandy,  At  various  heights  on  the  steep  sides  of  these  hills 
are  outstanding  pillars  and  pinnacles  of  a  very  hard  and  com- 
pact chalk,  as  at  Tournedos  and  Elbeuf,  near  Rouen,  which  evi- 
dently owe  their  shape  to  the  power  of  the  waves.  (See  Figures.) 


Fig.  168. 


Fig.  169. 


Tournedos,  above  Rouen. 


Fig.  170. 


Elbeuf.  Elbeuf. 

Needles  and  grooved  pillars  of  chalk. 


Some  small  columns  near  Elbeuf  exhibit  parallel  and  horizontal 
grooves  scooped  out  of  the  columns  at  different  heights.  (See 
Fig.  170.)  These  greatly  resemble  certain  limestone  pillars, 
described  by  Captain  Bayfield,  in  the  Mingan  islands  in  the  gulf 
of  St.  Lawrence.  There  is  evidence  there  of  the  coast  having 
been  upheaved  at  successive  periods,  so  that  parallel  ranges  of 
sea  beaches,  with  recent  shells,  have  been  laid  dry,  terrace 
above  terrace.  At  heights  corresponding  to  the  beaches  the  iso- 
lated masses  of  calcareous  rock  retain  the  marks  worn  by  the 
waves.  These  marks  probably  indicate  pauses  in  the  upheaving 
process,  during  which  the  sea  had  a  considerable  time  to  wear 


PART  II.     CHAPTER  XV. 


195 


Chalk  Needles 


Cretaceous  Group. 


away  the  stone,  as  well  as  to  throw  up  a  beach  at  the  same 
level.* 

The  needles  of  the  Isle  of  Wight,  and  the  Old  Harry  Rocks 
of  the  coast  of  Dorsetshire,  are  well  known  to  those  who  have 
examined  the  chalk  cliffs  of  the  South  of  England.  Besides  the 
inland  columns  in  Normandy,  above  described,  there  are  others 
more  recently  formed  on  the  sea  coast  of  that  same  country. 

Fig.  171. 


Needle  and  Arch  of  Etretat,  in  the  chalk  cliffs  of  Normandy, 
Height  of  Arch  100  feet.    (Passy.)f 

If  we  inquire  at  what  period  the  emergence  and  denudation 
'of  the  cretaceous  rocks  took  place,  we  shall  find  that  it  occurred 
in  great  part  after  the  deposition  of  various  rnarine  tertiary  forma- 
tions, so  that  both  the  cretaceous  and  tertiary  beds  were  upraised 
together.  The  greatest  elevation  which  the  chalk  reaches  in 
England,  is  the  summit  of  Inkpen  Beacon,  in  Berkshire,  which 
is  1011  feet  above  the  sea;  but  marine  deposits  of  the  same  age 
attain  an  elevation  of  8000  feet  in  the  Alps  and  Pyrenees. 
These  may  have  partly  emerged  during  the  cretaceous  period, 
just  as  the  coral  reefs  in  some  regions  of  the  Pacific  are  grow- 
ing in  one  spot,  while  other  portions  of.  the  same  have  been 
uplifted  by  subterranean  forces,  and  converted  into  land. 

Difference  between  the  chalk  of  the  north  and  south  of 
Europe. — By  the  aid  of  the  three  tests  of  relative  age,  namely, 
superposition,  mineral  character,  and  fossils,  the  geologist  has 
been  enabled  to  refer  to  the  same  cretaceous  period  certain  rocks 
in  the  north  and  south  of  Europe,  which  differ  greatly  both  in 
their  fossil  contents,  and  in  their  mineral  composition  and  struc- 
ture. 

If  we  attempt  to  trace  the  cretaceous  deposits  from  England 

*  Captain  Bayfield,  Geol.  Trans.,  Second  Series,  vol.  v.  p.  94.  Also  Principles 
of  Geology,  Index,  "  Niapisca  island." 

t  Seine-Inferieure,  p.  142.  and  plate  6.  fig.  1, 


196 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Different  Character  of  Chalk  in  South  of  Europe. 


Fig.  172. 


and  France  to  the  countries  bordering  the  Mediterranean,  we 
perceive,  in  the  first  place,  that  the  chalk  and  green-sand  in  the 
neighbourhood  of  London  and  Paris,  form  one  great  continuous 
mass,  the  strait  of  Dover  being  a  trifling  interruption,  a  mere 
valley  with  chalk  cliffs  on  both  sides.  We  then  observe  that  the 
main  body  of  the  chalk  which  surrounds  Paris  stretches  from 
Tours  to  near  Poitiers,  (see  the  annexed  map  (Fig.  172.),  in 
which  the  shaded  part  represents  chalk.) 

Between  Poitiers  and  La  Ro- 
chelle,  the  space  marked  A  on  the 
map  separates  two  regions  of 
chalk.  This  space  is  occupied  by 
the  oolite  and  certain  other  forma- 
tions older  than  the  chalk,  and  has 
been  supposed  by  M.  E.  de  Beau- 
mont to  have  formed  an  island  in 
the  cretaceous  sea.  South  of  this 
space  we  again  meet  with  a  for- 
mation which  we  at  once  recog- 
nize by  its  mineral  character  to  be 
chalk,  although  there  are  some 
places  where  the  rock  becomes 
oolitic.  The  fossils  also  are  upon 
the  whole  very  similar,  although" 
some  new  forms  now  begin  to  ap- 
pear in  abundance,  which  are  rare 
or  wholly  unknown  further  to  the 
north.  Among  these  may  be  men- 
tioned many  Hippurites,  Sphasru- 
lites,  and  other  members  of  that 


great  family  of  mollusca  called  Rudistes  by  Lamarck,  to  which 
nothing  analogous  has  been  discovered  in  the  living  creation. 
Although  very  uncommon  in  England,  one  species  of  this  family 
has  been  discovered  in  our  chalk. 


Fiff.  174. 


Fig.  175. 


Hippurites  Mortoni,  Mantell.    Maidstone,  Kent. 
Diameter  one-seventh  of  nat.  size. 


PART  II.     CHAPTER  XV. 


197 


Different  Character  of  Chalk  in  South  of  Europe. 


Fig.  173.  Two  individuals  deprived  of  their  opercula,  adhering  together 

174.  Same  seen  from  above. 

175.  Transverse  section  of  part  of  the  wall  of  the  shell,  magnified  to  show 

the  structure. 

176.  Vertical  section  of  the  same. 

On  the  side  where  the  shell  is  thinnest,  there  is  one  external  furrow  and  cor- 
responding internal  ridge,  a.  6.  Figs.  173,  174.  ;  but  they  are  usually  less  promi- 
nent than  in  these  figures.  This  species  has  been  referred  to  Hippurites,  but 
does  not,  I  believe,  fully  agree  in  character  with  that  genus.  I  have  never  seen 
the  opercular  piece,  or  valve,  as  it  is  called  by  those  conchologists  who  regard  the 
Rudisles  as  bivalve  mollusca. 

But  this  family,  which  is  so  feebly  represented  in  England 
and  the  north  of  France,  becomes  quite  characteristic  of  rocks 
of  the  cretaceous  era  in  the  south  of  France,  Spain,  Greece,  and 
other  countries  bordering  the  Mediterranean. 


Fig.  177. 


'.  O^Sar%alve  of  H.  radios*.  \  Lower  chalk>  South  of  France" 


Fig.  178. 


Fig.  179. 


Spharulites  agariciformis. 


Hippurites  organisans.   Desm.  Pyrenees. 


Between  the  region  of  chalk  last  mentioned  in  which  Perigeux 
is  situated,  and  the  Pyrenees,  the  space  B  intervenes.  (See  Map.) 

Here  the  tertiary  strata  cover,  and  for  the  most  part  conceal, 
the  cretaceous  rocks,  except  in  some  spots  where  they  have 
been  laid  open  to  view  by  the  denudation  of  the  newer  forma- 
tions. In  these  places  they  are  seen  still  preserving  the  form  of 
a  white  chalky  rock,  which  is  filled  in  part  with  grains  of  green- 
sand.  Even  as  far  south  as  Tercis,  on  the  Adour,  near  Dax,  it 
retains  this  character  where  I  have  examined  it,  and  where  M. 
Grateloup  has  found  in  it  Ananchytes  ovata  (Fig.  158.),  and 
other  fossils  of  the  English  chalk,  together  with  Hippurites. 
When  we  arrive  at  Bayonne  and  the  Pyrenees,  the  cretaceous 


198 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Different  character  of  Chalk  in  South  of  Europe. 


formation,  although  still  exhibiting  some  of  the  same  mineralo- 
gical  peculiarities,  is  nevertheless  greatly  changed.  Its  calca- 
reous division  consists  for  the  most  part  of  compact  crystalline 
marble,  often  full  of  nummulites  (see  Fig.  180.),  and  those  por- 

Fig.  180. 


Nummulite  limestone ;  Peyrehorade,  Pyrenees. 

a  External  surface  of  one  of  the  nummulites,  of  which  longitudinal  sections  are 

seen  in  the  limestone. 
b  Transverse  section  of  same.  % 

tions  which  may  be  imagined  to  represent  the  green-sand,  are 
composed  of  shales,  grits,  and  micaceous  sand-stone,  containing 
impressions  of  marine  plants,  together  with  lignite  and  coal. 
There  are  also  beds  of  red  sandstone  and  conglomerate,  belong- 
ing to  the  same  group.  These  rocks  ascend  gradually  into  the 
highest  part  of  the  Pyrenees,  and  cross  over  into  Spain,  where 
the  cretaceous  system  assumes  a  character  still  more  unlike  that 
of  northern  Europe. 

Here,  as  on  the  north  side  of  the  Pyrenees,  the  most  conspicu- 
ous fossils  are  .hippurites,  sphaBrulites,  and  nummulites.  The 
last-mentioned  fossil,  so  called  from  its  resemblance  to  a  piece 
of  money,  is  a  genus  of  mollusca,  very  abundant  in  the  tertiary 
strata  of  Northern  Europe ;  but  only  met  with  in  chalk  in  the 
South  of  Europe. 

So  many  species  and  genera  of  shells  now  wanting  in  our 
northern  seas,  are  frequent  in  the  Mediterranean,  that  we  need 
not  be  surprised,  when  following  from  north  to  south  the  deposits 
of  the  old  cretaceous  sea,  at  finding  similar  modifications  in  or- 
ganic forms. 

The  cretaceous  rocks  in  the  Alps,  Italy,  Greece,  and  Asia 
Minor,  are  distinct  in  like  manner  from  the  type  of  that  forma- 
tion in  the  North  of  Europe ;  yet  their  age  in  most  of  these 
countries  can  be  clearly  ascertained,  partly  by  following  them 
continuously  from  the  north  in  the  manner  above  described ;  and 
partly  by  their  position  below  the  tertiary,  and  above  the  oolitic 
strata. 


PART  II.    CHAPTER  XV.  199 

Ancient  Submergence  of  South  of  Europe Terminology. 

We  learn  from  the  researches  of  M.  M.  Boblaye,  and  Virlet, 
(hat  the  cretaceous  system  in  the  Morea,  is  composed  of  compact 
and  lithographic  limestones  of  great  thickness ;  also  of  granular 
limestones,  with  jasper ;  and  in  some  districts,  as  in  Messenia, 
a  puddingstone,  with  a  siliceous  cement  more  than  1600  feet  in 
thickness,  belongs  to  the  same  group.f 

It  is  evident,  observe  these  geologists,  from  the  great  range 
of  the  hippurite  and  nummulite  limestone,  that  the  South  of 
Europe  was  occupied  at  the  cretaceous  period  by  an  immense 
sea,  which  extended  from  the  Atlantic  Ocean  into  Asia,  and  com- 
prehended the  southernmost  part  of  France,  together  with  Spain, 
Sicily,  part  of  Italy,  and  the  Austrian  Alps,  Dalmatia,  Albania, 
a  portion  of  Syria,  the  isles  of  the  ./Egean,  coasts  of  Thrace,  and 
the  Troad. 

In  proportion,  therefore,  as  we  enlarge  the  sphere  of  our  re- 
searches,  we  may  find  in  the  strata  of  one  era,  the  mineralogical 
counterparts  of  the  rocks,  which,  in  a  single  country  like  Eng- 
land, may  characterise  successive  periods.  Thus,  the  grits, 
sandstone,  and  shale  with  coal,  of  the  Pyrenees  have  actually 
been  mistaken  by  skilful  miners  for  the  ancient  carboniferous 
group  of  England  and  France.  In  like  manner  the  cretaceous 
red  marl  and  salt  of  northern  Spain  have  been  regarded  as  the 
same  as  our  new  red  and  saliferous  sandstone ;  and  the  litho- 
graphic limestone  of  the  Morea  might  be  confounded  with  the 
oolite  of  Solenhofen  in  Germany. 

The  beginner,  perhaps,  on  hearing  these  facts,  may  object  to 
the  term  cretaceous,  as  applied  to  the  rocks  of  the  southern  re- 
gion in  which  there  is  no  chalk.  But  the  term  green-sand 
would  have  been  equally  inappropriate  as  a  general  name  for 
this  group ;  and  that  of  hippurite  and  nummulite  limestone,  how- 
ever well  suited  to  the  Mediterranean  region,  would  be  inappli- 
cable to  the  chalk  of  the  north.  Scarcely  any  designation  would 
remain  unexceptionable  as  we  enlarge  the  bounds  of  our  know- 
ledge, and  we  must  therefore  be  content  to  retain  many  ancient 
names,  as  simply  expressing  the  mineral  or  palseontological  cha- 
racters of  rocks  in  the  country  where  they  were  first  studied. 

*Bull.  de  la  Soc.  Geol.  de  France,  torn.  iii.  p.  149. 


200  LYELL'S  ELEMENTS  OF  GEOLOGY. 


Wealdeii  Group. 


CHAPTER  XVI. 


WEALDEN  GROUP. 

The  Wealden,  including  the  Weald  clay,  Hastings  sand,  and  Purbeck  beds — 
Intercalated  between  two  marine  formations  —  Fossil  shells  freshwater,  with  a 
few  marine — Cypris — Fish — Reptiles — Birds — Plants — Section  showing  passage 
of  Wealden  beneath  chalk — Junction  of  Wealden  and  Oolite — Dirt-bed — Theory 
of  gradual  subsidence — Proofs  that  the  Wealden-strata,  notwithstanding  their 
thickness,  may  have  been  formed  in  shallow  water  —  Geographical  extent  of 
Wealden — Bray  near  Beauvais — Relation  of  the  Wealden  to  the  Lower  Green- 
sand  and  Oolite. 

BENEATH  the  cretaceous  rocks  in  the  S.  E.  of  England,  a 
freshwater  formation  is  found,  called  the  Wealden,  which,  al- 
though it  occupies  a  small  area  in  Europe,  as  compared  to  the 
chalk,  is  nevertheless  of  great  interest,  as  being  intercalated  be- 
tween two  marine  formations.  It  is  composed  of  three  minor, 
groups,  of  which  the  aggregate  thickness  in  some  places  cannot 
be  less  than  800  feet.*  These  subdivisions  are, 

Thickness. 
1st.  Weald  clay,  sometimes  including  thin  beds  of  sand  and 

shelly  limestone *. 140  to  280  ft. 

2d.  Hastings  sand,  in  which  occur  some  clays  and  calcareous 

grits ;— between 400  and  500  ft. 

3d.  Purbeck  beds,  consisting  of  various  kinds  of  limestones  and 

marls about  250  ft. 

To  all  these  subdivisions,  the  common  name  of  the  Wealden 
has  been  given,  because  they  may  be  best  studied  in  part  of 
Kent,  Surrey,  and  Sussex,  called  the  Weald. 

We  have  seen  that  the  fossils  of  the  chalk  and  green-sands 
which  repose  upon  the  Wealden  are  all  marine,  and  the  species 
numerous ;  and  the  same  remark  applies  to  the  Portland  stone 
and  other  members  of  the  Oolitic  series  which  lie  immediately 
beneath  (see  Fig.  181.).  But  in  the  Wealden  itself,  although 
the  fossils  are  abundant  as  to  quantity,  the  number  of  different 
species  is  comparatively  small,  and  by  far  the  greater  part  of 
them  show  that  they  were  deposited  in  a  freshwater  lake,  or  es- 
tuary communicating  with  the  sea.f 


Dr.Fitton,  Geol.  Trans,  vol.  iv.  p.  320.  Second  Series.  tlbid.  p.  104. 


PART  II.     CHAPTER  XVI. 


201 


Wealden  Group Fossils. 

Fig.  181. 


marine 


freshwater 


Weald  Clay,     ) 
Hastings  sand,  >  Wealden. 
Purbeck  beds,    ; 


Oolite. 


Position  of  the  Wealden  between  two  marine  formations. 

Fossils  of  the  Wealden. — The  shells  of  this  formation  are 
almost  exclusively  of  fluviatile  or  lacustrine  genera,  such  as  Me- 
lanopsis,  Paludina,  Neritina,  Cyclas,  Unio,  and  others.  The  indi- 
viduals are  sometimes  in  such  profusion,  that  the  surface  of  each 
thin  layer  of  marl  or  clay  is  covered  with  the  valves  of  Cyclas,  and 
whole  beds  of  limestone  are  almost  entirely  composed  of  Palu- 
dinse.  Intermixed  with  these  freshwater  shells,  there  are  a  few 
which  seem  to  mark  the  occasional  presence  of  salt  water,  as  for 
example,  a  species  of  Bulla,  together  with  an  Oyster,  and  the 
Exogyra,  a  genus  of  unimuscular  bivalves  allied  to  the  oyster  (see 
Fig.  182.).  The  conclusion  to  be  drawn  from  the  presence  of  a 
Corbula  (see  Fig.  183.)  and  Mytilus  is  more  doubtful;  for  al- 


Fig.  182. 


Fig.  183. 


Exogyra  bulla.    Fitton. 


Corbula  alata.  Fitton.    Magnified. 


though  these  genera  are  for  the  most  part  marine,  still  there  is  a 
Mytilus  living  in  the  Danube,  and  one  species  of  Corbula  in- 
habits the  river  La  Plata,  in  South  America,  as  well  as  the  ad- 
joining sea,  while  another  is  common  to  the  Caspian,  and  the 
rivers  Don  and  Wolga.  But  admitting  all  these  to  have  been 
marine,  they  by  no  means  outweigh  the  evidence,  both  of  a  posi- 
tive and  negative  kind,  derived  from  shells  in  favour  of  the  fresh- 
water origin  of  the  Wealden.  In  no  part  of  this  deposit  do  we 
meet  with  ammonites,  belemnites,  terebratulse,  corals,  sea-urchins, 
or  other  testacea  and  zoophytes  so  characteristic  of  the  chalk 
above,  or  the  oolite  below  the  Wealden. 


202 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Wealden  Group Fossils. 


Fig.  184. 


Fig.  185. 


Fig.  186. 


Cypris  spini- 
gera,  Fitton. 


Cypris  Valdensis,  Fitton.  (C.faba, 
Min.  Con.  485.) 


Ci/pris  tuberculata, 
Fitton. 


Fig.  187. 


Shells  of  the  Cypris,  an  animal  allied  to  the  Crustacea,  and 
before  mentioned  (p.  47.)  as  abounding  in  lakes  and  ponds,  are 
also  plentifully  scattered  through  the.  clays 
of  the  Wealden,  sometimes  producing  like 
plates  of  mica,  a  thin  lamination  (see  Fig. 
187.).  Similar  cypriferous  marls  are  found 
in  the  lacustrine  tertiary  beds  of  Auvergne, 
and  in  recent  deposits  of  shell  marl. 

The  fishes  of  the  Wealden  belong  partly 
to  the  genera  Pyenodus  and  Hybodus,  forms 
common  to  the  Wealden  and  Oolite  (see  Fig. 
225.);  but  the  teeth  and  scales  of  a  species  of  Lepidotus  are 
most  widely  diffused  (see  Fig.  188.).  The  general  form  of  these 
fish  was  that  of  the  carp  tribe,  although  perfectly  distinct  in  ana- 
tomical character,  and  more  allied  to  the  pike.  The  whole  body 
was  covered  with  large  rhomboidal  scales  very  thick,  and  having 
the  exposed  part  covered  with  enamel.  Most  of  the  species  of 

Fig.  188. 


a.  palate  and  teeth. 


Lepidotus  Mantelli.    Agass.  Wealden. 
b.  side  view  of  teeth. 


c.  scale. 


this  genus  are  supposed  to  have  been  either  river  fish,  or  inhab- 
itants of  the  coasts,  having  not  sufficient  powers  of  swimming  to 
advance  into  the  deep  sea. 

Among  the  remains  of  vertebrata,  those  of  reptiles  form  the 
most  remarkable  feature.  Some  of  them  belong  to  tortoises, 
such  as  the  Trionyx  and  Emys,  genera  now  occurring  in  fresh- 


PART  II.    CHAPTER  XVI.  203 

Wealden  Group Fossil  Plants. 

water  in  tropical  regions.  Of  Saurian  lizards  there  are  at  least 
five  genera ;  the  Crocodile,  Plesiosaur,  Megalosaur,  Iguanodon, 
and  Hyteosaur.  The  Iguanodon,  of  which  the  remains  were 
first  discovered  by  Mr.  Mantell,  was  an  herbivorous  reptile,  and 
was  regarded  by  Cuvier  as  more  extraordinary  than  any  with 
which  he  was  acquainted  ;  for  the  teeth,  though  bearing  a  great 
analogy  to  the  modern  Iguanas  which  now  frequent  the  tropical 
woods  of  America  and  the  West  Indies,  exhibit  many  striking 
and  important  differences  (see  Fig.  190.).  It  appears  that  they 
have  been  worn  by  mastication ;  whereas  the  existing  herbivo- 
rous reptiles  clip  and  gnaw  off  the  vegetable  productions  on 

which  they  feed,  but  do  not  chew 

Teeth  of  Iguanodon.  them.      Their   teeth,   when   worn, 

Fig.  190.  present  an  appearance  of  having 
been  chipped  off,  and  never,  like  the 
fossil  teeth  of  the  Iguanodon,  have  a 

Fig".  189.  §§llli}.)    ^at  grounc'  surface,  (see  Fig.  189.), 

resembling  the  grinders  of  herbivo- 
rous mammalia.  From  the  large 
bones,  found  in  great  numbers  near 
these  teeth,  and  fairly  presumed  to 
belong  to  the  same  animal,  it  is 
computed  that  the  entire  length  of 
Crown  of  tooth  in  Pointed  tooth  of  this  reptile  could  not  have  been  less 

adult,  worn  down,     a  young  animal.      ,1 

Mantcii.  Manteii.          than  seventy  feet. 

The  bones  of  birds  of  the  order 

GrallaB  or  Waders  have  been  discovered  by  Mr.  Mantell  in  the 
Wealden,  and  appear  to  be  the  oldest  well  authenticated  exam- 
ples of  fossils  of  this  class  hitherto  found  in  Great  Britain.*  But 
no  portion  of  the  skeleton  of  a  mammiferous  quadruped  has  yet 
been  met  with.  • 

The  vegetable  remains,  which  are  numerous,  exhibit  many 
characters  of  a  tropical  flora,  some  being  allied  to  the  living 
genera  Cycas  and  Zamia,  (see  Fig.  194.,)  others  to  large  Equi- 
seta.  There  are  also  Coniferse  allied  to  Araucaria,  and  other 
genera  of  warm  climates,  (see  Fig.  191.)  besides  numerous 
ferns.  (See  Fig.  192.) 

Passage  of  Wealden  beneath  Chalk. — It  has  been  already 
seen  that  the  chalk  and  green-sand  have  an  aggregate  thickness 
of  1000  or  sometimes  1500  feet.  It  is  therefore  a  wonderful 
fact  that  after  penetrating  these  rocks,  we  come  down  upon  a 
subjacent  freshwater  formation '  from  800  to  1000  feet  in  thick- 
ness. The  order  of  superposition  is  clear,  for  we  see  the  weald 

*  Mantell.  Proceedings  Geol.  Soc.  vol.  ii.  p.  203. 


204 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Position  of  Wealden. 


Fig.  191. 


Cone  from  the  Isle  of  Purbeck, 
resembling  the  Dammara  of 
the  Moluccas.  Fitton. 


Sphenopteris  gracilis  (Fitton),  from 

near  Tunlridge  Wells. 
a.  portion  of  the  same  magnified. 


clay  passing  beneath  the  green-sand  in  various  parts  of  Surrey, 
Kent,  and  Sussex ;  and  if  we  proceed  from  Sussex  westward  to 
the  Vale  of  Wardour,  we  there  again  observe  the  same  forma- 
tion occupying  the  same  relative  position,  and  resting  on  the 


Fig.  193. 


Vale  of  Wardour.     Wilts. 


Hants. 


Sussex. 


Wealden. 


Wealden. 


oolite.  (See  Fig.  193.)  Or  if  we  pass  from  the  base  of  the 
south  downs  in  Sussex,  and  cross  to  the  Isle  of  Wight,  we  there 
again  meet  with  the  same  series  reappearing  beneath  the  green- 
sand,  and  we  cannot  doubt  that  the  beds  are  prolonged  subterra- 
neously,  as  indicated  by  the  dotted  lines  in  Fig.  194. 


Isle  of  Wight. 


It  has  been  already  suggested  that,  during  the  accumulation 
of  the  green-sand,  there  was  a  gradual  sinking  down  and  sub- 
mersion of  land,  by  which  the  wide  open  sea  of  the  chalk  was 
produced.  But  the  position  of  the  Wealden  points  still  more 
forcibly  to  such  a  conclusion,  and  especially  the  appearances 
exhibited  at  the  point  of  junction  of  the  wealden,  and  the  oolitic 


PART  n.   CHAPTER  xvi.  205 

Portland  Dirt-Bed Fossil  Forest  in  Isle  of  Portland. 

formation  on  which  it  rests.  First,  in  regard  to  its  junction  with 
the  superincumbent  lower  green-sand,  the  beds  of  this  last,  says 
Dr.  Fitton,  repose  in  the  south-east  of  England,  conformably 
upon  those  of  the  subjacent  weald  clay.  There  is  no  indication 
of  disturbance :  "  To  all  appearance  the  change  from  the  depo- 
sition of  the  freshwater  remains  to  that  of  the  marine  shells, 
may  have  been  effected  simply  by  a  tranquil  submersion  of  the 
land  to  a  greater  depth  beneath  the  surface  of  the  waters."* 

Portland  dirt-bed  and  proofs  of  subsidence. — But  when  we 
examine  the  contact  of  the  Purbeck  beds,  or  inferior  division  of 
the  wealden,  with  the  Portland  stone,  or  upper  member  of  the 
oolite,  some  very  singular  phenomena  are  observed.  Between 
the  two  formations,  the  marine  and  the  freshwater,  there  inter- 
venes in  Portland  a  layer  of  dark  matter,  called  by  the  quarry- 
men  the  "  Dirt,"  or  "  Black  dirt,"  which  appears  evidently  to 
have  been  an  ancient  vegetable  soil.  It  is  from  twelve  to  eighteen 
inches  thick,  is  of  a  dark  brown  or  black  colour,  and  contains  a 
large  proportion  of  earthy  lignite.  Through  it  are  dispersed 
rounded  fragments  of  stone,  from  three  to  nine  inches  in  diame- 
ter, in  such  numbers  that  it  almost  deserves  the  name  of  gravel. 
Many  silicified  trunks  of  coniferous  trees,  and  the  remains  of 
plants  allied  to  the  Zamia  and  Cycas  are  buried  in  this  dirt-bed. 
(See  figure  of  living  Zamia.) 

Fig.  195. 


Zamia  spiralis;  Southern  Australia-! 


These  plants  must  have  become  fossil  on  the  spots  where  they 
grew.  The  stumps  of  the  trees  stand  erect  for  a  height  of  from 
one  to  three  feet,  and  even  in  one  instance  to  six  feet,  with  their 
roots  attached  to  the  soil  at  about  the  same  distances  from  one 

*  Geol.  of  Hastings,  p.  28.  t  See  Flinder's  Voyage. 

S 


206      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Fossil  Forest  in  Isle  of  Portland. 

another  as  the  trees  in  a  modern  forest.*  The  carbonaceous 
matter  is  most  abundant  immediately  around  the  stumps,  and 
round  the  remains  of  fossil  Cycadete.^ 

.  Besides  the  upright  stumps  above  mentioned,  the  dirt-bed  con- 
tains the  stems  of  silicified  trees  laid  prostrate.  These  are  partly 
sunk  into  the  black  earth,  and  partly  enveloped  by  a  calcareo- 
siliceous  slate  which  covers  the  dirt-bed.  The  fragments  of  the 
prostrate  trees  are  rarely  more  than  three  or  four  feet  in  length ; 
but  by  joining  many  of  them  together,  trunks  have  been  restored 
having  a  length  from  the  root  to  the  branches  of  from  20  to  23 
feet,  the  stems  being  undivided  for^l?  or  20  feet,  and  then  forked. 
The  diameter  of  these  near  the  roots  is  about  one  foot.ij:  Root- 
shaped  cavities  were  observed  by  Professor  Henslow  to  descend 
from  the  bottom  of  the  dirt-bed  into  the  subjacent  Portland  stone, 
so  that  the  uppermost  beds  of  the  Portland  limestone,  though 
now  solid,  were  in  a  soft  and  penetrable  state  when  the  trees 
giew.§ 

The  thin  layers  of  calcareous  slate  (Fig.  196.)  were  evidently 

Fig.  196. 

freshwater  calcareous 
slate. 

dirt-bed  and  ancient 
forest. 

marine  Portland 
stone. 

V s 

Section  in  Isle  of  Portland,  Dorset.    (Buckland  and  De  la  Beche.) 

deposited  tranquilly,  and  would  have  been  horizontal  but  for  the 
protrusion  of  the  stumps  of  the  trees,  around  the  top  of  each  of 
which  they  form  hemispherical  concretions. 

The  dirt-bed  is  by  no  means  confined  to  the  island  of  Port- 
land, but  is  seen  in  the  same  relative  position  in  a  cliff  east  of 
Lul worth  Cove,  in  Dorsetshire,  where,  as  the  strata  have  been 
disturbed,  and  are  now  inclined  at  an  angle  of  45°,  the  stumps 
of  the  trees  are  also  inclined  at  the  same  angle  in  an  opposite 
direction — a  beautiful  illustration  of  a  change  in  the  position  of 

*  Mr.  Webster  first  noticed  the  erect  position  of  the  trees  and  described  the 
Dirt-bed.  The  account  here  given  is  drawn  from  Dr.  Buckland  and  Mr.  De  la 
Beche,  Geol.  Trans.,  Second  Series,  vol.  iv.  p.  1.;  Mantell,  Geol.  of  S.  E.  of 
England,  p.  336. ;  and  Dr.  Fitton,  Geol.  Trans.,  Second  Series,  vol.  iv.  p.  220. 

t  Fitton,  ibid.  pp.  220,  221.  $  Ibid. 

$  Buckland  and  De  la  Beche,  Geol.  Trans.,  Second  Series,  vol.  iv.  p.  16. 


PART  II.     CHAPTER  XVI. 


207 


Fossil  Forest  in  Isle  of  Portland. 


Fig.  197. 


freshwater  calcareous 

slate, 
dirt-bed. 


Portland  stone  of  ma- 
rine formation. 


Section  in  cliff  east  of  Lulworth  Cove.    (Buckland  and  De  la  Beche.) 

beds  originally  horizontal.  (See  Fig.  197.)  Traces  of  the  dirt- 
bed  have  also  been  observed  by  Dr.  Buckland,  about  two  miles 
north  of  Thame,  in  Oxfordshire ;  and  by  Dr.  Fitton,  in  the  cliffs 
of  the  Boulonnois,  on  the  French  coast:  but,  as  might  be 
expected,  this  freshwater  deposit  is  of  limited  extent  when  com- 
pared to  most  marine  formations. 

From  the  facts  above  described,  we  may  infer,  first,  that  the 
superior  beds  of  the  oolite,  which  are  full  of  marine  shells, 
became  dry  land,  and  covered  by  a  forest,  throughout  a  portion 
of  the  space  now  occupied  by  the  south  of  England,  the  climate 
being  such  as  to  admit  the  growth  of  the  zamia  and  cycas. 
2dly.  This  land  at  length  sank  down  and  was  submerged  with 
its  forests  beneath  a  body  of  freshwater,  from  which  sediment 
enveloping  fluviatile  shells  was  deposited.  3dly.  "  The  regular 
and  uniform  preservation  of  this  thin  bed  of  black  earth  over  a 
distance  of  many  miles,  shows  that  the  change  from  dry  land  to 
the  state  of  a  freshwater  lake  or  estuary,  was  not  accompanied 
by  any  violent  denudation,  or  rush  of  water,  since  the  loose 
black  earth,  together  with  the  trees  which  lay  prostrate  on  its 
surface,  must  inevitably  have  been  swept  away  had  any  such 
violent  catastrophe  then  taken  place."* 

The  dirt-bed  has  been  described  above  in  its  most  simple  form, 
but  in  some  sections  the  appearances  are  more  complicated. 
The  forest  of  the  dirt-bed  was  not  everywhere  the  first  vegeta- 
tion which  grew  in  this  region.  Tw^j  other  beds  of  carbona- 
ceous clay,  one  of  them  containing  CycadetB  in  an  upright  posi- 
tion, have  been  found  below  it,f  which  implies  other  oscillations 
in  the  level  of  the  same  ground,  and  its  alternate  occupation  by 
land  and  water  more  than  once.  There  must  have  been,  first, 


*  Buckland  and  De  la  Beche,  Geol.  Trans.,  Second  Series,  vol.  iv.  p.  16. 
t  Fitton,  Geol.  Trans.,  Second  Series,  vol.  iv.  p.  S23. 


208  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Wealden  Strata  formed  in  Shallow  Water. 

the  sea  in  which  the  corals  and  shells  of  the  oolite  grew ;  then, 
land,  which  supported  a  vegetable  soil  with  Cycadese ;  then,  a 
lake  or  estuary,  in  which  freshwater  strata  were  deposited ;  then, 
again,  land,  on  which  other  Cycadese  and  a  forest  of  dicotyledo- 
nous trees  flourished ;  then,  a  second  submergence  under  fresh- 
water, in  which  the  wealden  strata  were  gradually  formed ;  and, 
finally,  in  the  cretaceous  period,  a  return  over  the  same  space 
of  the  ocean. 

To  imagine  such  a  series  of  events  will  appear  extravagant 
and  visionary  to  some  who  are  not  aware  that  similar  changes 
occur  in  the  ordinary  course  of  nature ;  and  that  large  areas 
near  the  sea  are  now  subject  to  be  laid  dry,  and  then  submerged, 
after  remaining  years  covered  with  houses  and  trees.* 

In  some  of  these  modern  revolutions,  such  as  have  been  wit- 
nessed in  the  delta  of  the  Indus,  in  Cutch,  we  have  instances  of 
land  being  permanently  laid  under  the  waters,  both  of  the  river 
and  the  sea,  without  the  soil  and  its  shrubs  being  swept  away ; 
but  such  preservation  of  an  ancient  soil  must  be  a  rare  exception 
to  the  general  rule,  for  it  would  be  destroyed  by  denuding  waves 
and  currents,  unless  the  land  sank  suddenly  down  to  a  great 
depth,  or  unless  its  form  was  such  as  to  exclude  the  free  ingress 
of  the  sea. 

Notwithstanding  the  enormous  thickness  of  the  wealden,  ex- 
ceeding in  some  places  perhaps  1000  feet,  there  are  many 
grounds  for  believing  that  the  whole  of  it  was  a  deposit  in  water 
of  moderate  depth,  and  often  extremely  shallow.  This  idea  may 
seem  startling  at  first,  yet  such  would  be  the  natural  consequence 
of  a  gradual  and  continuous  sinking  of  the  ground  in  an  estuary 
or  bay,  into  which  a  great  river  discharged  its  turbid  waters. 
By  each  foot  of  subsidence,  the  fundamental  rock,  such  as  the 
Portland  oolite,  would  be  depressed  one  foot  farther  from  the  sur- 
face of  the  ocean ;  but  the  bay  would  not  be  deepened  if  new 
strata  of  mud  and  sand  should  raise  the  bottom  one  foot.  On 
the  contrary,  such  sand  and  mud  might  be  frequently  laid  dry 
at  low  water,  or  overgrown  for  a  season  by  a  vegetation  proper 
to  marshes.  At  different  heights  in  the  Hastings  Sand  in  the 
middle  of  the  Wealden,  we  find  again  and  again  slabs  of  sand- 
stone with  a  strong  ripple-mark,  and  between  these  slabs  beds 
of  clay  many  yards  thic£.  In  some  places,  as  at  Stammerham, 
near  Horsham,  there  are  indications  of  this  clay  having  been 
exposed  so  as  to  dry  and  crack  before  the  next  layer  was  thrown 
down  upon  it.  The  open  cracks  .in  the  clay  have  served  as 

*  For  an  account  of  recent  movements  of  land  attended  by  such  conse- 
quences, see  Principles  of  Geology,  Index,  "  Cutch,"  "  Sindree,"  &c. 


PART  II.    CHAPTER  XVI. 


209 


Wealden  Group Its  Geographical  Extent. 


198. 


Underside  of  slab  of  sandstone,  about  one  yard  in  diame 
ter ;  Stammerham,  Sussex. 


moulds,  of  which 
casts  have  been  taken 
in  relief,  and  which 
are,  therefore,  seen 
on  the  lower  surface 
of  the  sandstone*  (see 
Fig.  198.). 

Near  the  same 
place  a  reddish  sand- 
stone occurs  in  which 
are  innumerable  tra- 
ces of  a  fossil  vegeta- 
ble, apparently  Sphe- 
nopteris,  the  stems 
and  branches  of  which 
are  disposed  as  if  the 
plants  were  standing 
erect  on  the  spot  where  they  originally  grew,  the  sand  having 
been  gently  deposited  upon  and  around  them  ;  and  similar  ap- 
pearances have  been  remarked  in  other  places  in  this  formation.! 
In  the  same  division  also  of  the  wealden,  at  Cuckfield,  is  a  bed 
of  gravel  or  conglomerate,  consisting  of  water-worn  pebbles  of 
quartz  and  jasper,  with  rolled  bones  of  reptiles.  These  must 
have  been  drifted  by  a  current,  probably  in  water  of  no  great 
depth. 

The  occasional  presence  of  oysters  in  the  Purbeck  limestone, 
and  throughout  the  Hastings  sand  and  Weald  clay,  proves  that 
the  waters  of  the  sea  sometimes  found  access  into  the  estuary  ,:{: 
whether  in  consequence  of  subsidence,  or  in  seasons  when  the 
body  of  freshwater  was  lessened  in  volume. 

Geographical  extent.  —  The  Wealden  strata  have  been  traced 
about  200  English  miles  from  west  to  east,  from  Lul  worth  Cove 
to  near  Boulogne,  in  France,  and  about  220  miles  from  north- 
west to  south-east,  from  Whitchurch,  in  Buckinghamshire,  to 
Beauvais,  in  France.  If  the  formation  be  continuous  throughout 
this  space,  which  is  very  doubtful,  it  does  not  follow  that  the 
whole  was  contemporaneous  ;  because  in  all  likelihood  the  phy- 
sical geography  of  the  region  underwent  frequent  change  through- 
out the  whole  period,  and  the  estuary  may  have  altered  its  form, 
and  even  shifted  its  place.  Yet  some  modern  deltas  are  of  vast 
size,  as  for  example,  that  of  the  newly  discovered  Quorra,  or 

*  Observed  by  Mr.  Mantell  and  myself,  in  1831. 
t  Mantell,  Geol.  of  S.E.  of  England,  p.  244. 
\  Fitton,  Geol.  Trans.,  2d  Ser.,  vol.  iv,  p.  321. 


210  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Origin  of  Wealden  Group. 

Niger,  in  Africa,  which  stretches  into  the  interior  for  more  than 
170  miles,  and  occupies,  it  is  supposed,  a  space  of  more  than  300 
miles  along  the  coast;  thus  forming  a  surface  of  more  than 
25,000  square  miles,  or  equal  to  about  one  half  of  England.* 

I  have  stated  that  the  Wealden  has  been  observed  near  Beau- 
vais,  in  France ;  and  the  locality  is  marked  in  the  section,  at  p. 
196.  It  is  called  "the  country  of  Bray;"  and  resembles  in 
structure  the  English  Weald  beneath  the  north  and  south  downs. 
In  a  similar  manner  the  green-sand  crops  out  from  beneath  the 
chalk,  and  fresh- water  strata  from  beneath  the  green-sand.  One 
member  of  the  series,  a  fine  whitish  sand,  contains  impressions 
of  ferns,  considered  by  M.  Adolphe  Brongniart  as  identical  with 
Lonchopteris  Mantelli,  a  plant  frequently  found  in  the  Wealden. 
I  examined  part  of  the  valley  of  Bray  in  company  with  M. 
Graves,  in  1833,  and  I  observed  that  the  sand  last  mentioned, 
with  its  vegetable  remains,  was  intercalated  between  two  sets  of 
marine  strata,  containing  trigonise,  and  referred  by  French  geol- 
ogists to  the  lower  green-sand.  In  the  same  country  of  Bray, 
and  associated  with  the  same  formation,  is  a  limestone  resem- 
bling the  Purbeck  marble,  and  containing  a  Paludina  which 
seems  specifically  identical  with  that  of  Purbeck. 

If  it  be  asked  where  the  continent  was  placed  from  the  ruins 
of  which  the  Wealden  strata  were  derived,  and  by  the  drainage 
of  which  a  great  river  was  fed,  we  are  half  tempted  to  speculate 
on  the  former  existence  of  the  Atlantis  of  Plato.  The  story  of 
the  submergence  of  an  ancient  continent,  however  fabulous  in 
history,  may  be  true  as  a  geological-  event.  Its  disappearance 
may  have  been  gradual ;  and  we  need  not  suppose  that  the  rate 
of  subsidence  was  hastened  at  the  period  when  the  displacement 
of  a  great  body  of  freshwater  by  the  cretaceous  sea  took  place. 
Suppose  the  mean  height  of  the  land  drained  by  the  river  of  the 
Wealden  estuary  to  have  been  no  more  than  800  or  1000  feet; 
in  that  case,  all  except  the  tops  of  the  mountains  would  be 
covered  as  soon  as  the  fundamental  oolite  and  the  dirt-bed  were 
sunk  down  about  1000  feet  below  the  level  which  they  occupied 
when  the  forest  before-mentioned  was  growing.  Towards  the 
close  of  the  period  of  this  subsidence,  both  the  sea  would  en- 
croach and  the  river  diminish  in  volume  more  rapidly ;  yet  in 
such  a  manner,  that  we  may  easily  conceive  the  sediment  at 
first  washed  into  the  advancing  sea  to  have  resembled  that  pre- 
viously deposited  by  the  river  in  the  estuary.  In  fact,  the 
upper  beds  of  the  Wealden,  and  the  inferior  strata  of  the  lower 

*Fitton,  Geoi.  of  Hastings,  p.  58. ;  who  cites  Lander's  Travels. 


PART  II.     CHAPTER  XVI.  211 


Relative  Age  of  the  Wealden  Group. 


green-sand,  are  not  only  conformable,  but  of  similar  mineral 
composition. 

It  is  also  a  remarkable  fact,  that  the  same  Iguanodon  Man- 
telli  which  is  so  conspicuous  a  fossil  in  the  Wealden,  has  recently 
been  discovered  near  Maidstone,  in  the  overlying  Kentish  rag,  or 
marine  limestone  of  the  lower  green-sand.  Hence  we  may  infer 
that  some  of  the  saurians  which  inhabited  the  country  of  the 
great  river,  continued  to  live  when  part  of  the  country  had 
become  submerged  beneath  the  sea.  Thus,  in  our  own  times, 
we  may  suppose  the  bones  of  large  alligators  to  be  frequently 
entombed  in  recent  freshwater  strata  in  the  delta  of  the  Ganges. 
But  if  part  of  that  delta  should  sink  down  so  as  to  be  covered 
by  the  sea,  marine  formations  might  begin  to  accumulate  in  the 
same  space  where  freshwater  beds  had  previously  been  formed; 
and  yet  the  Ganges  might  still  pour  down  its  turbid  waters  in 
the  same  direction,  and  carry  the  carcasses  of  the  same  species 
of  alligator  to  the  sea,  in  which  case  their  bones  might  be  in- 
cluded in  marine  as  well  as  in  subjacent  freshwater  strata. 

Age  of  the  Wealden. — Some  geologists  have  classed  the 
Wealden  as  a  member  of  the  cretaceous  group,  while  others  have 
considered  it  as  more  nearly  connected  with  the  antecedent 
oolitic  deposits ;  nor  is  it  easy  to  decide  which  opinion  is  prefer- 
able, because  the  organic  remains  of  the  cretaceous  and  oolitic 
groups  are  marine,  while  those  of  the  interposed  Wealden  are 
almost  all  freshwater.  The  testacea  and  plants  of  the  latter 
appear  as  yet  to  be  specifically  distinct  from  those  of  any  other 
formation ;  but  if  we  examine  the  reptiles,  it  appears  that  the 
Megalosaurus  Bucklandi  is  common  to  the  Oolite  and  Wealden, 
the  teeth  and  bones  of  this  great  Saurian  occurring  both  in  the 
limestone  of  Stonesfield  and  in  the  Hastings  sand. 

There  are  also  some  generic  forms,  both  of  reptiles  and  fish, 
common  to  the  Oolite  and  Wealden,  and  not  yet  discovered  in 
the  Chalk.  VertebraB,  for  example,  of  the  Plesiosaurus  are  not 
confined  to  the  oolite  and  lias,  but  have  been  also  found  in  the 
Wealden ;  and  the  Lepidotus,  a  gejius  of  fish  very  characteristic 
of  the  Wealden,  is  unknown  in  the  cretaceous  group,  while  it  is 
abundant  in  the  oolitic  series. 

On  the  other  hand,  the  same  species  of  Iguanodon  has  been 
already  mentioned  as  decidedly  common  to  the  Wealden  and 
green-sand. 

In  Scotland,  and  in  different  parts  of  the  Continent,  marine 
deposits  have  been  found  which  are  supposed  to  have  been  coe- 
val with  the  Wealden,  and  which  are  intermediate  in  fossil  cha- 
racters as  in  position  between  the  Cretaceous  and  Oolitic  sys- 


212  LYELL'S  ELEMENTS  OF  GEOLOGY. 


Absence  of  Mammalia  in  the  Wealden. 


terns.*  They  may  have  been  contemporaneous  deltas  of  other 
rivers  flowing  from  the  same  ancient  continent. 
~-  Absence  of  mammalia. — Among  the  numerous  fossils  of  the 
Wealden,  no  remains  of  mammalia  have  been  hitherto  detected  , 
whereas  we  should  naturally  expect,  on  examining  the  deposits 
recently  formed  at  the  mouths  of  the  Quorra,  Indus,  or  Ganges, 
to  find,  not  only  the  bones  of  birds,  and  of  amphibious  and  land 
reptiles,  but  also  those  of  such  warm-blooded  quadrupeds  as  fre- 
quent the  banks  of  rivers,  or,  like  the  hippopotamus,  inhabit 
their  waters.  Would  not  the  same  current  of  water  which  drifted 
down  and  rolled  the  bones  of  the  lizards,  tortoises,  and  fish  of 
the  Wealden,  have  also  swept  down  into  the  delta  some  frag- 
ments at  least  of  mammiferous  bones,  had  any  animals  of  the 
highest  class  been  then  in  existence  ?  As  a  general  rule,  indeed, 
we  cannot  lay  much  stress  on  mere  negative  evidence ;  and  it 
may  be  well  to  notice,  that  although  so  many  teeth  of  the  Iguan- 
odon  have  been  collected,  it  is  only  of  late  that  a  single  small 
portion  of  a  jaw  of  one  of  these  gigantic  lizards  was  obtained. 
Perhaps,  in  like  manner,  some  bone  or  tooth  of  a  fossil  quadru- 
ped will  one  day  be  found.  We  may  at  least  say,  that  we  have 
at  present  no  example  of  a  continent  covered  with  a  luxuriant 
vegetation,  and  forests  inhabited  by  large  saurians,  both  aquatic 
and  terrestrial,  and  by  birds,  yet  at  the  same  time  entirely  desti- 
tute of  warm-blooded  quadrupeds.  The  nearest  analogy  to  this 
state  of  things  is  that  of  New  Zealand ;  and  this  fact  will  be 
more  particularly  alluded  to  in  the  sequel.  (See  p.  256.) 

In  conclusion  I  may  remark,  that  from  the  time  of  the  com- 
mencement of  the  Wealden,  to  far  on  in  the  cretaceous  period, 
we  have  signs  of  subsidence,  and  consequent  diminution  of  land. 
But  after  the  chalk  was  formed,  or  during  the  tertiary  periods, 
we  have,  on  the  contrary,  proofs  of  an  increase  of  land  in 
Europe.  But  we  must  not  extend  these  generalizations  to  the 
whole  surface  of  the  globe ;  for  other  large  areas  may  have  been 
growing  more  and  more  continental  during  the  cretaceous,  and 
more  and  more  oceanic  during  the  tertiary  periods,  the  direction 
of  the  prevailing  subterranean  movement  being  reversed. 

*  See  Fitton,  Geol.  Trans.,  Second  Series,  vol.  iv.  p.  328.,  and  his  references. 


PART  II.     CHAPTER  XVII.  213 


Divisions  of  the  Oolite. 


CHAPTER  XVII. 


OOLITE   AND   LIAS. 

Subdivisions  of  the  Oolitic  group — Fossil  shells — Corals  in  the  calcareous  divi- 
sions only — Buried  forest  of  Encrinites  in  Bradford  clay— Changes  in  organic 
life  during  accumulation  of  Oolites — Characteristic  fossils — Signs  of  neighbour- 
ing land  and  shoals — Supposed  cetacea  in  Oolite — Oolite  of  Yorkshire  and  Scot- 
land. 

OOLITE.  —  Below  the  freshwater  group  last  described,  or, 
where  this  is  wanting,  immediately  beneath  the  Cretaceous  for- 
mation, a  great  series  of  marine  strata,  commonly  called  "  the 
Oolite,"  occurs  in  many  parts  of  Europe.  This  group  has  been 
so  named,  because,  in  England  and  other  places  where  it  was 
first  examined,  the  limestones  belonging  to  it  had  an  oolitic 
structure.  (See  p.  27.)  These  rocks  occupy  in  England  a  zone 
which  is  nearly  thirty  miles  in  average  breadth,  and  extends 
across  the  island,  from  Yorkshire  on  the  north-east,  to  Dorset- 
shire on  the  south-west.*  Their  mineral  characters  are  not  uni- 
form throughout  this  region ;  but  the  following  are  the  names 
of  the  principal  subdivisions  observed  in  the  central  and  south- 
eastern parts  of  England : — 

OOLITE. 

TT__D1.  f  a.  Portland  stone  and  sand. 
Upper{  6.  Kimmeridge  clay. 
Tiyfj^i    {  c"  Coral  rag. 
Mlddle  U  Oxford  clay. 

(  e.  Cornbrash  and  Forest  Marble. 
Lower  <f.  Great  Oolite  and  base  of  Fullers'  earth. 

£  g.  Inferior  Oolite. 
The  Lias  then  succeeds  to  the  Inferior  Oolite. 

The  upper  oolitic  system  of  the  above  Table  has  usually  the 
Kimmeridge  clay  for  its  base,  and  the  middle  oolitic  system  the 
Oxford  clay.  The  lower  system  reposes  on  the  Lias,  an  argillo- 
calcareous  formation,  which  some  include  in  the  lower  oolite, 'but 
which  will  be  treated  of  separately  in  the  next  chapter.  Many 
of  these  subdivisions  are  distinguished  by  peculiar  organic 

*  For  details  respecting  this  formation  in  England,  see  Conybeare  and  Phil- 
lips's  Geology,  chap.  iii. 


214  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Oolite  Group Fossil  Corals. 

remains ;  and  though  varying  in  thickness,  may  be  traced  in 
certain  directions  for  great  distances,  especially  if  we  compare 
the  part  of  England  to  which  the  above-mentioned  type  refers 
with  the  north-west  of  France,  and  the  Jura  mountains,  which 
separate  that  country  from  Switzerland,  and  in  which,  though 
distant  above  400  geographical  miles,  the  analogy  to  the  English 
type  above  mentioned  is  more  perfect  than  in  Yorkshire  or  Nor- 
mandy. 

To  enter  upon  a  systematic  description  of  this  complicated 
series  of  strata  would  require  many  chapters ;  the  following 
facts,  therefore,  are  selected  from  a  multitude  of  others,  with  a 
view  of  illustrating  the  origin  of  the  oolitic  rocks,  and  of  show- 
ing the  state  of  organic  life  arid  geographical  condition  of  part 
of  the  globe  when  they  were  formed. 

In  almost  all  the  minor  divisions  enumerated  in  the  above 
Table,  Ammonites  and  Belemnites  are  found,  (see  Figs.  213. 
215.)  but  of  species  different  from  those  of  the  cretaceous  period. 
The  ammonites  are  of  various  sizes,  from  the  size  of  a  small 
carriage-wheel  to  less  than  an  inch  diameter. 

It  is  not  uncommon  to  find  belemnites  in  different  members  of 
the  series,  with  full-grown  serpulae  attached  to  them.  As  these 
shells,  like  the  bone  of  the  cuttle-fish,  so  often  thrown  on  our 
shores,  were  internal,  it  is  clear,  that  after  the  death  of  the 
cephalopod  the  belemnite  remained  for  some  time  unburied  at  the 
bottom  of  the  sea,  so  that  the  serpulse  grew  upon  it. 

These  cephalopoda,  swimming  about  in  the  open  sea,  left  their 
shells  to  be  imbedded  indifferently  in  whatever  sediment  was 
then  in  the  course  of  deposition,  whether  calcareous  or  argilla- 
ceous. But  the  corals  are  almost  entirely  confined  to  the  lime- 
stones, and  are  wanting  in  the  dense  formations  of  interposed 
clay,  as  also  in  the  lias,  these  zoophytes  requiring,  not  only  car- 
bonate of  lime  for  their  support,  and  clear  water,  but  a  bottom 
remaining  Tor  years  unchanged,  either  by  the  shifting  of  sand  or 
the  accession  of  fresh  sediment. 

In  the  Upper  Oolite  of  England,  corals  are  rare,  although  one 
species  is  found  plentifully  at  Tisbury,  in  Wiltshire,  in  the  Port- 
land sand,  converted  into  flint  and  chert,  the  original  calcareous 
matter  being  replaced  by  silex.  (Fig.  199.)  One  of  the  lime- 
stones of  the  Middle  Oolite  has  been  called  the  "  Coral  Rag," 
because  it  consists,  in  part,  of  continuous  beds  of  petrified  corals, 
for  the  most  part  retaining  the  position  in  which  they  grew  at  the 
bottom  of  the  sea.  They  belong  chiefly  to  the  genera  Caryo- 
phyllia  (Fig.  200.),  Agaricia,  and  Astrea,  and  sometimes  form 
masses  of  coral  fifteen  feet  thick.  These  coralline  strata  extend 
through  the  calcareous  hills  of  the  N.  W.  of  Berkshire,  and 


PART  II.     CHAPTER  XVII. 


215 


Corals  of  Oolite. 


Eig.  199. 


Fig.  200. 


Columnaria  oblonga,  Blainv. 
Upper  Oolite,  Tisbury. 


Caryophyllia  annularis,  Parkin. 
Coral  rag,  Steeple  Ashton. 


north  of  Wilts,  and  again  recur  in  Yorkshire,  near  Scarborough. 
Although  the  name  of  coral  rag  has  been  thus  appropriated, 
there  are  portions  of  the  lower  oolite,  as  for  example  the  Great 
and  Inferior  Oolite  (/.  g.  Table,  p.  213.),  which  are  equally  en- 
titled in  many  places  to  be  called  coralline  limestones.  Thus 
the  Great  Oolite  near  Bath  contains  various  corals,  among  which 
the  Eunomia  radiata  (Fig.  201.)  is  very  conspicuous,  single 

Fig.  201. 


Eunomia  radiata,  Lamouroux. 

a.  section  transverse  to  the  tubes. 

6.  vertical  section,  showing  the  radiation  of  the  tubes. 

c.  portion  of  interior  of  tubes  magnified,  showing  striated  surface. 

individuals  forming  masses  several  feet  in  diameter ;  and  having 
probably  required,  like  the  large  existing  brain-coral  (Meandrina) 
of  the  tropics,  many  centuries  before  their  growth  was  completed. 
Different  species  of  Crinoideans,  or  stone-lilies,  are  also  com- 
mon in  the  same  rocks  with  corals ;  and,  like  them,  must  have 


216 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Bradford  Encrinites. 


enjoyed  a  firm  bottom,  where  their  root,  or  base  of  Attachment, 
remained  undisturbed  for  years  (c.  Fig.  202.)      Such  fossils, 

Fig.  202. 


Jlpiocrinites  rotundas,  or  Pear  Encrinite ;  Miller.     Fossil  at  Bradford,  Wilts. 
a.  Stem  of  Apiocrinitea,  and  one  of  the  articulations,  natural  size. 
5.  Section  at  Bradford  of  great  oolite  and  overlying  clay,  containing  the  fossil 
encrinites.     See  text. 

c.  Three  perfect  individuals  of  the^  Apiocrinite,  represented  as  they  grew  on 

the  surface  of  the  Great  Oolite. 

d.  Body  of  the  Apiocriniles  rotundus. 

therefore,  are  almost  confined  to  the  limestones  ;  but  an  excep- 
tion occurs  at  Bradford,  near  Bath,  where  they  are  enveloped  in 
clay.  In  this  case,  however,  it  appears  that  the  solid  upper  sur- 
face of  the  "  Great  Oolite"  had  supported,  for  a  time,  a  thick 
submarine  forest  of  these  beautiful  zoophytes,  until  the  clear  and 
still  water  was  invaded  by  a  current  charged  with  mud,  which 
threw  down  the  stone-lilies,  and  broke  most  of  their  stems  short 
off  near  the  point  of  attachment.  The  stumps  still  remain  in 
their  original  position  ;  but  the  numerous  articulations  once  com- 
posing the  stem,  arms,  and  body  of  the  zoophyte,  were  scattered 
at  random  through  the  argillaceous  deposit  in  which  some  of 
them  now  lie  prostrate.  These  appearances  are  represented  in 
the  section  £,  Fig.  202.,  where  the  darker  strata  represent  the 
Bradford  clay,  a  member  of  the  Forest  marble  (e.  Table,  p.  213.). 
The  upper  surface  of  the  calcareous  stone  below  is  completely 
incrusted  over  with  a  continuous  pavement,  formed  by  the  stony 
roots  or  attachments  of  the  Crinoidea ;  and  besides  this  evidence 
of  the  length  of  time  they  had  lived  on  the  spot,  we  find  great 
numbers  of  single  vertebra?,  or  circular  plates  of  the  stem  and 
body  of  the  encrinite,  covered  over  with  serpulse.  Now  these 
serpulse  could  only  have  begun  to  grow  after  the  death  of  some 
of  the  stone-lilies,  parts  of  whose  skeletons  had  been  strewed 
over  the  floor  of  the  ocean  before  the  irruption  of  argillaceous 


PART  II.     CHAPTER  XVH.  217 

Peculiar  Fossils. 

mud.  In  some  instances  we  find  that,  after  the  parasitic  serpu- 
\3d  were  full  grown,  they  had  become  incrusted  over  with  a  coral, 
called  Berenicea  diluviana ;  and  many  generations  of  these 
polyps  had  succeeded  each  other  in  the  pure  water  before  they 
became  fossil. 

Fig.  203. 


a.  Single  vertebra,  or  articulation  of  an  Encrinite  overgrown  with  serpulae  and 

corals.     Natural  size.    Bradford  clay. 
I.  Portion  of  the  same  magnified,  showing  the  coral  Berenicea  diluviana 

covering  one  of  the  serpulse. 

We  may,  therefore,  perceive  distinctly  that,  as  the  pines  and 
cycadeous  plants  of  the  ancient  Portland  Forest  were  killed  by 
submergence  under  fresh-water,  and  soon  buried  under  muddy 
sediment,  so  an  invasion  of  argillaceous  matter  put  a  sudden  stop 
to  the  growth  of  the  Bradford  Encrinites,  and  led  to  their  preser- 
vation in  marine  strata.* 

Such  differences  in  the  fossils  as  distinguish  the  calcareous 
and  argillaceous  deposits  from  each  other,  would  be  described  by 
naturalists  as  arising  out  of  a  difference  in  the  stations  of  spe- 
cies ;  but  besides  these,  there  are  variations  in  the  fossils  of  the 
higher,  middle,  and  lower  part  of  the  oolitic  series,  which  must 
be  ascribed  to  that  great  law  of  change  in  organic  life  by  which 
distinct  assemblages  of  species  have  been  adapted,  at  successive 
geological  periods,  to  the  varying  conditions  of  the  habitable  sur- 
face. In  a  single  district  it  is  difficult  to  decide  how  far  the 
limitation  of  species  to  certain  minor  formations  has  been  due  to 
the  local  influence  of  stations,  or  how  far  it  has  been  caused  by 
time,  or  the  creative  and  destroying  law  above  alluded  to. v  But 
we  recognize  the  reality  of  the  last-mentioned  influence,  when 
we  contrast  the  whole  oolitic  series  of  England  with  that  of  parts 

*  For  a  fuller  account,  of  these  Encrinites,  see  Buckland's  Bridgewater  Trea- 
tise, vol.  i.  p.  429. 
T 


218 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Fossils  of  Oolite  Group. 


of  the  Jura,  Alps,  and  other  distant  regions,  where  there  is 
scarcely  any  lithological  resemblance ;  and  yet  some  of  the  same 
fossils  remain  peculiar  in  each  country  to  the  Upper,  Middle,  and 
Lower  Oolite  formations  respectively.  Mr.  Thurmann  has  shown 
how  remarkably  this  fact  holds  true  in  the  Bernese  Jura,  although 
the  argillaceous  divisions,  so  conspicuous  in  England,  are  feebly 
represented,  and  some  entirely  wanting. 

Amongst  the  characteristic  fossils  of  the  Upper  Oolite,  may 
be  mentioned  the  Ostrea  deltoidea  (Fig.  205.),  found  in  the  Kim- 
meridge  clay  throughout  England  and  the  north  of  France,  and 


FOSSILS  OF  THE  OOLITE. 
Fig.  205. 


Fig.  204. 


r 


Fig.  206. 


GrypJuea  virgula.  Ostrea  deltoidea. 

Upper  Oolite. 


Fig.  207. 


Trigonia  gibbosa. 

a  the  binge. 
Portland  Oolite,  Tisbury. 

Fig.  208. 


hieroglyphica. 
Coral  rag. 


Jferinaa  Goodhallii,  Fit  ton. 
Coral  rag,  Weymoutli. 


also  in  Scotland,  near  Brora.  The  Gryphcsa  virgula  (Fig. 
204.),  also  met  with  in  the  same  clay  near  Oxford,  and  so  abun- 
dant in  the  upper  oolite  of  parts  of  France  as  to  have  caused  the 
deposit  to  be  termed  "  marnes  a  gryphees  virgule."  Near  Cler- 
rnont,  in  Argonne,  a  few  leagues  from  St.  Menehould,  these  indu- 


PART  II.     CHAPTER  XVII. 


219 


Fossils  of  Oolite  Group. 


Fig.  209. 


Fig.  210. 


Cast  of  Diceras  arietina. 
Coral  rag. 


Cidaris  coronata. 
Coral  rag. 


rated  marls  crop  out  from  beneath  the  gault;  and,  on  decomposing, 
leave  the  surface  of  every  ploughed  field  literally  strewed  over 
with  fossil  oysters. 

One  of  the  limestones  of  the  Jura,  referred  to  the  age  of  the 
English  coral  rag,  has  been  called  "  Nerinsean  limestone"  (Cal- 
caire  a  Nerinees)  by  M.  Thirria;  Nerincea  being  an  extinct 
genus  of  univalve  shells,  much  resembling  the  Cerithium  in 
external  form,  and  peculiar  to  the  oolitic  period.  The  annexed 
section  (Fig.  207.)  shows  the  curious  form  of  the  hollow  part  of 
each  whorl,  and  also  the  perforation  which  passes  up  the  middle 
of  the  columella.  N.  Goodhallii  (Fig.  208.)  is  another  English 
species  of  the  same  genus,  from  a  formation  which  seems  to  form 
a  passage  from  the  Kimmeridge  clay  to  the  coral  rag.* 

A  division  of  the  oolite  in  the  Alps,  regarded  by  most  geolo- 
gists as  coeval  with  the  English  coral  rag,  has  been  often  named 
"  Calcaire  a  Dicerates,"  or  "  Diceras  limestone,"  from  its  con- 
taining abundantly  a  bivalve  shell  (see  Fig.  209.)  of  a  genus 
allied  to  the  Chama. 

Among  the  characteristic  shells  of  the  Inferior  Oolite,  I  may 
instance  Terebratula  spinosa  (Fig.  211.),  Pholadomya  fdi- 


Fig.  211. 


Fig.  212. 


Terebratula  spinosa. 
Inferior  Oolite. 


a  Pholadomya  fidicula.    Inferior  Oolite. 

b  Heart-shaped  anterior  termination  of  the  same. 


*  Fitton,  Geol.  Trans.,  Second  Series,  vol.  iv.  pi.  23,  fig.  12. 


220 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Fossils  of  Oolite  Group. 


Fisr.  213. 


JBelemnites  hastatus.    Inferior  Oolite. 

Fig.  214.  Fig.  215. 


Orbicula  reflexa,  Sow. 

a.  upper  valve. 

b.  lower  or  attached  valve,  and 

showing  part  of  the  upper. 

Fig.  216. 


Ammonites  slriatulus,  Sow. 
Inferior  Oolite  and  Lias. 


Fig.  217. 


Terebratula  digona, 
Inferior  Oolite. 


Ostrea  Marshii. 
Middle  and  Lower  Oolite. 


cula  (Fig.  212.),  Belemnites  hastatus  (Fig.  213.),  and  Terebra- 
tula digona  (Fig  216.) 

As  illustrations  of  shells  having  a  great  vertical  range,  I  may 
allude  to  Trigonia  gibbosa  (Fig.  205.),  which  abounds  in  the 
Portland  stone  of  Wiltshire,  and  the  Inferior  Oolite  of  York- 
shire.* Also  Ostrea  Marshii  (Fig.  217.),  common  to  the 
Cornbrash  of  Wilts  and  the  Inferior  Oolite  of  Yorkshire ;  and, 
lastly,  Orbicula  reflexa  (Fig.  214.)  and  Ammonites  striatulus 
(Fig.  215.),  fossils  common  to  the  Inferior  Oolite  and  Lias. 

Such  facts  by  no  means  invalidate  the  general  rule,  that  cer- 
tain fossils  are  good  chronological  tests  of  geological  periods ; 
but  they  serve  to  caution  us  against  attaching  too  much  import- 
ance to  single  species,  some  of  which  may  have  a  wider,  others 
a  more  confined  vertical  range.  We  have  before  seen  that,  in 


*  See  Williamson,  Proceedings  Geol.  Soc.  No.  47. 


PART  II.     CHAPTER  XVII.  221 

Oolite  Group Signs  of  Land. 

some  of  the  tertiary  formations,  some  species  occur  both  in  the 
older  and  newer  groups,  yet  these  groups  may  be  distinguishable 
from  one  another  by  a  comparison  of  the  whole  assemblage  of 
fossil  shells  proper  to  each. 

Signs  of  neighbouring  land  and  shoals, — The  corals  and 
shells  above  alluded  to,  and  the  fish,  Crustacea,  and  other  accom- 
panying fossils,  sufficiently  attest  the  marine  origin  of  the  oolitic 
strata  in  general.  Yet  there  are  frequent  signs  of  shallow  water 
and  of  neighbouring  land ;  and  these  are  the  more  worthy  of 
attention,  as  they  by  no  means  diminish  as  we  proceed  down- 
wards to  the  inferior  parts  of  the  oolitic  series.  Had  the  bottom 
of  the  sea  in  Europe  been  unmoved  during  the  entire  oolitic 
period,  the  first,  or  oldest  beds  of  the  oolite,  must  have  been 
accumulated  in  the  deepest  water,  the  middle  oolite  in  water  of 
less  depth,  and  the  upper  in  the  shallowest  of  all.  The  appear- 
ances about  to  be  described  militate  against  this  conclusion.  The 
Kimmeridge  clay,  in  the  Upper  Oolite,  consists,  in  great  part,  of 
a  bituminous  shale,  sometimes  forming  an  impure  coal  several 
hundred  feet  in  thickness.  In  some  places  in  Wiltshire  it  much 
resembles  peat ;  and  the  bituminous  matter  may  have  been,  in 
part  at  least,  derived  from  the  decomposition  of  vegetables.  But 
as  impressions  of  plants  are  rare  in  these  shales,  which  contain 
ammonites,  oysters,  and  other  marine  shells,  the  bitumen  may 
perhaps  be  of  animal  origin.  The  occurrence,  however,  of  fossil 
wood  in  the  Upper  Oolite  shows  that  there  were  then  lands  from 
which  plants  were  drifted  into  the  sea. 

The  celebrated  lithographic  stone  of  Solenhofen,  in  Bavaria, 
belongs  to  one  of  the  upper  divisions  of  the  oolite,  and  affords  a 
remarkable  example  of  the  variety  of  fossils  which  may  be  pre- 
served under  favourable  circumstances,  and  what  delicate  impres- 
sions of  the  tender  parts  of  certain  animals  and  plants  may  be 
retained  where  the  sediment  is  of  extreme  fineness.  Although 
the  number  of  testacea  in  this  slate  is  small,  and  the  plants  few, 
and  those  all  marine,  Count  Munster  had  determined  no  less  than 
237  species  of  fossils  when  I  saw  his  collection  in  1833;  and 
among  them  no  less  than  seven  species  of  flying  lizards,  or 
pterodactyls,  six  saurians,  three  tortoises,  sixty  species  of  fish, 
forty-six  of  Crustacea,  and  twenty-six  of  insects.  These  insects, 
among  which  is  a  libellula,  or  dragon-fly,  must  have  been  blown 
out  to  sea,  probably  from  the  same  land  to  which  the  flying 
lizards,  and  other  contemporaneous  reptiles,  resorted. 

In  one  of  the  upper  members  of  the  Inferior  Oolite  of  Eng- 
land the  ripple-mark  is  distinctly  seen  throughout  a  considerable 
thickness  of  thin  fissile  beds  of  a  coarsely  oolitic  limestone.  The 
rippled  slabs  are  used  for  roofing,  and  have  been  traced  over  a 

T* 


222  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Oolite  Group Stonesfield  Slate. 

broad  band  of  country  from  Bradford,  in  Wilts,  to  Tetbury,  in 
Gloucestershire.  These  calcareous  slabs,  or  tile-stones,  are  sepa- 
rated from  each  other  by  thin  seams  of  clay,  which  have  been 
deposited  upon  them,  and  have  taken  their  form,  preserving  the 
undulating  ridges  and  furrows  of  the  sand  in  such  complete 
integrity,  that  the  impressions  of  small  footsteps,  apparently  of 
crabs,  which  walked  over  the  soft  wet  sands,  are  still  visible.  In 
the  same  stone  the  claws  of  crabs,  fragments  of  echini,  broken 
shells,  pieces  of  drift  wood,  and  other  signs  of  a  neighbouring 
beach,  are  observed. 

The  slate  of  Stonesfield  has  lately  been  shown  by  Mr.  Lons- 
dale  to  lie  at  the  base  of  the  Inferior  Oolite.     It  is  an  oolitic 
shelly  limestone,  only  six  feet  thick,  but  very  rich  in  organic 
remains.     It  contains  some  pebbles  of  a  rock  very  similar  to 
itself,  and  with  them  the  fossil  remains  of  belemnites,  trigonia?, 
and  other  marine  shells.    Besides  fragments  of  wood, 
Fig.  218.     which  occur  in  all  parts  of  the  oolitic  group,  there  are 
many  impressions  of  ferns,  cycadese,  and  other  ter- 
restrial plants.     Several  insects  also,  and  among  the 
rest,  the  wing-covers  of  beetles,    are  perfectly  pre- 
served, (see  Fig.  218.)  some  of  them  approaching 
nearly  to  the  genus  Buprestis.*     The  remains,  also, 
of  many  genera  of  reptiles,  such    as  Plesiosaurus, 
Crocodile,  and  Pterodactyl,  have  been  discovered  in 
the  same  limestone ;  and,  what  is  still  more  remark- 
able,  the  jaws  of  at  least  two  species  of  mammiferous 
Buprestis;     quadrupeds,   allied   to    the   Didelphys,   or   opossum. 

Stonesfield.     «l  J.       . ,         «,     ,    Al  ,  i          A  i  /• 

1  hese  fossils  afford  the  only  example  yet  known  of 
terrestrial  mammalia  in  rocks  of  a  date  anterior  to  the  Eocene 
period. 

This  exception  is  the  more  deserving  of  notice,  because  even 
no  cetacea  have  as  yet  been  observed  in  any  secondary  strata, 
although  certain  bones,  from  the  great  oolite  of  Enstone,  near 
Woodstock,  in  Oxfordshire,  have  been  cited,  on  the  authority  of 
Cuvier,  as  referable  to  this  class.  Dr.  Buckland,  who  has  stated 
this  in  his  late  Bridgewater  Treatise,f  has  had  the  kindness  to 
send  me  the  supposed  ulna  of  a  whale,  in  order  that  Mr.  Owen 
might  examine  into  its  claims  to  be  considered  as  cetaceous.  It 
is  the  opinion  of  that  eminent  comparative  anatomist,  that  it  can- 
not have  belonged  to  the  cetacea,  because  the  fore-arm  in  these 
marine  mammalia  is  invariably  much  flatter,  and  devoid  of  all 
muscular  depressions  and  ridges,  one  of  which  is  so  prominent  in 
the  middle  of  this  bone.  (See  Fig.  219.)  In  saurians,  on  the 

*  See  Buckland's  Bridgewater  Treatise.  t  Vol.  i.  p.  115. 


PART  II.     CHAPTER  XVII. 


223 


No  Fossil  Cetacea  in  Oolite  Group. 


Fig.  219. 


Bone  of  a  reptile,  formerly  supposed  to  be  the  ulna  of  a  Cetacean  ;  from  the 
Oolite  of  Enstone,  near  Woodstock. 

contrary,  such  ridges  exist  for  the  attachment  of  muscles ;  and 
to  some  animal  of  that  class  the  bone  is  probably  referable. 

Oolite  of  Yorkshire  and  Scotland. — North  of  the  Humber, 
in  Yorkshire,  the  Inferior  Oolite  assumes  a  form  very  different 
from  that  which  distinguishes  it  in  the  south.  It  may  there  be 
called  a  coal  formation,  as  it  contains  much  vegetable  matter, 
and  coal,  interstratified  with  sand  and  sandstones.  The  high 
state  of  preservation  and  number  of  the  plants  render  it  probable 
that  land  was  not  far  distant.  The  same  may  be  said  of  the 
oolitic  coal  of  Brora,  on  the  south-east  coast  of  Sutherland- 
shire,  in  Scotland,  where  the  Inferior  Oolite  contains  coal,  one 
bed  of  which  is  3£  feet  in  thickness.  The  plants  resemble  those 
in  the  Yorkshire  oolite,  and  a  great  number  of  the  associated 
marine  shells  and  other  fossils  are  the  same  ;*  but  the  mineral 
characters  of  the  sandstone,  shale,  and  calcareous  grit,  differ 
considerably. 


*  Murchison,  Geol.  Trans,,  vol.  ii.  Second  Series. 


-  \ 

224      LYELts  ELEMENTS  OF  GEOLOGY. 

.        ,x     "> 

'.Xj-1'  Lias. 


CHAPTER 

OOLITE  AND  LIAS — continued. 

Mineral  character  of  Lias — Name  of  Gryphite  limestone — Fossil  fish — Ich- 
thyodorulites — Reptiles  of  the  Lias — Ichthyosaur  and  Plesiosaur — Newly  disco- 
vered marine  Reptile  of  the  Galapagos  Islands — Sudden  death  and  burial  of 
fossil  animals  in  Lias — Origin  of  the  Oolite  and  Lias,  and  of  alternating  calcare- 
ous and  argillaceous  formations — ^Physical  geography — Vales  of  clay — Hills  and 
escarpments  of  limestone. 

LIAS. — The  English  provincial  name  of  Lias  has  been  very 
generally  adopted  for  a  formation  of  argillaceous  limestone,  marl, 
and  clay,  which  forms  the  base  of  the  oolite,  and  is  classed  by 
many  geologists  as  part  of  that  group.  They  pass,  indeed,  into 
each  other  in  some  places,  as  near  Bath,  a  sandy  marl  called 
the  marlstone  of  the  Lias  being  interposed,  and  partaking  of  the 
mineral  characters  of  the  upper  lias  and  inferior  oolite.  These 
last  mentioned  divisions  have  also  some  fossils  in  common,  such 
-p-  220  as  *ne  Avicula  intsquivalvis  (Fig.  220.) 

Nevertheless  the  Lias  may  be  traced  through- 
out a  great  part  of  Europe  as  a  separate  and 
independent  group,  of  considerable  thickness, 
varying  from  500  to  1000  feet,  containing 
many  peculiar  fossils,  and  having  a  very 
uniform  lithological  aspect.  Although  usu- 
a^7  conformable  to  the  oolite,  it  is  sometimes, 
Sow.  as  in  the  Jura,  unconformable.  Thus,  in 

the  environs  of  Lons-le-Saulnier,  for  instance,  the  strata  of  lias 
are  inclined  at  an  angle  of  about  45°,  while  the  incumbent  ooli- 
tic marls  are  horizontal. 

The  peculiar  aspect  which  is  most  characteristic  of  the  Lias 
in  England,  France,  and  Germany,  is  an  alternation  of  thin 
beds  of  limestone,  with  a  light  brown  weathered  surface,  sepa- 
rated by  dark-coloured  narrow  argillaceous  partings,  so  that  the 
quarries  of  this  rock,  at  a  distance,  assume  a  striped  and  riband- 
like  appearance.* 

Although  the  prevailing  colour  of  the  limestone  of  this  form- 
ation is  blue,  yet  some  beds  of  the  lower  lias  are  of  a  .yellowish 

*  Conyb.  and  Phil  p.  261. 


PART  II.     CHAPTER 

Fossils  of  the  Lias. 

white  colour,  and  have  been  called  white  lias.  In  some  parts 
of  France,  near  the  Vosges  mountains,  and  in  Luxembourg,  M. 
E.  de  Beaumont  has  shown  that  the  lias  containing  Gryph&a 
arcuata,  Plagiostoma  giganteum,  and  other  characteristic 
fossils,  becomes  arenaceous ;  and  around  the  Hartz,  in  West- 
phalia and  Bavaria,  the  inferior  parts  of  the  lias  are  sandy,  and 
sometimes  afford  a  building  stone  called  by  the  Germans  qua- 
dersandstein. 

The  name  of  Gryphite  limestone  has  sometimes  been  applied 
to  the  lias,  in  consequence  of  the  great  number  of  shells  which 
it  contains  of  a  species  of  oyster,  or  Gryphsea  (Fig.  221.). 
Many  cephalopoda,  also,  such  as  Ammonite,  Belemnite,  and 
Nautilus  (Fig.  222.),  prove  the  marine  origin  of  the  formation. 


Fig.  221. 


Fig.  222. 


Oryphcea  incurva,  Sow. 
(G.arcuata,  Lam.) 


Nautilus  truncatus,  Lias. 


The  fossil  fish  resemble  generically  those  of  the  oolite,  belong- 
ing all,  according  to  M.  Agassiz,  to  extinct  genera,  and  differing 
remarkably  from  the  ichthyolites  of  the  cretaceous  period. 
Among  them  is  a  species  of  Lepidotus  (L.  gigas,  Ag.)  Fig. 
202.),  which  is  found  in  the  lias  of  England,  France,  and  Ger- 

a  Fig.  223. 


Scales  of  Lepidotus  gigas,  Agas. 
a.  two  of  the  scales  detached. 

many.*     This  genus  was  before  mentioned  (p.  348.)  -as  occur- 
ing  in  the  Wealden,  and  is  supposed  to  have  frequented  both 


*  Agassiz,  Pois.  Fos.  vol.  ii.  tab.  28,  29. 


226 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Ichthyodorulites. 


rivers  and  coasts.     The  teeth  of  a  species  of  Acrodus,  also,  are 
very  abundant  in  the  lias  (Fig.  224.) 

Fig.  224. 


Acrodus  nobilis,  Agas.  tooth  ;  commonly  called  fossil  leach. 
Lias,  Lyme  Regis,  and  Germany. 

But  the  remains  of  fish  which  have  excited  more  attention 
than  any  others,  are  those  large  bony  spines  called  ichthyodo- 
rulites  (a.  Fig.  225.),  which  were  once  supposed  by  some  natu- 


Fig.  225. 


Hybodus  reticulatus,  Agas.  Lias,  Lyme  Regis. 
a.  Part  of  fin,  commonly  called  Ichthyodorulite. 
6-  Tooth. 

ralists  to  be  jaws,  and  by  others  weapons,  resembling  those  of 

the  living  Balistes  andSilurus;  but  which  M.  Agassiz  has  shown 

to  be  neither  the  one  nor  the  other.     The  spines,  in  the  genera 

Fig.  226.  ^ast  metinoned,  articu- 

late with  the  back- 
bone, whereas  there 
are  no  signs  of  any 
such  articulation  in 
the  ichthyodorulites. 
These  last  appear  to 
have  been  bony  spines 
which  formed  the  an- 
terior part  of  the  dor- 

Sal.  fin>  ^ke  tnat  °f  the 
living  genera  Cestra- 


CMmara  monstrosa* 
a.  Spine  forming  anterior  part  of  the  dorsal  fin. 


*  Agassiz,  Poissons  Fossiles,  vol.  iii.  tab.  C.  fig.  1. 


PART  II.     CHAPTER  XVIII.  227 

Reptiles  of  the  Lias. 

cion.and  Chimsera  (see  a.  Fig.  226.).  In  both  of  these  genera, 
the  posterior  concave  face  is  armed  with  small  spines  like  that 
of  the  fossil  Hybodus  (Fig.  225.),  one  of  the  shark  family  found 
fossil  at  Lyme  Regis.  Such  spines  are  simply  imbedded  in 
the  flesh,  and  attached  to  strong  muscles.  "  They  serve,"  says 
Dr.  Buckland,  "  as  in  the  Chimcera  (Fig.  226.),  to  raise  and 
depress  the  fin,  their  action  resembling  that  of  a  moveable  mast, 
raising  and  lowering  backwards  the  sail  of  a  barge."* 

Reptiles  of  the  Lias. — It  is  not,  however,  the  fossil  fish 
which  form  the  most  striking  feature  in  the  organic  remains  of 
the  Lias ;  but  the  reptiles,  which  are  extraordinary  for  their 
number,  size,  and  structure.  Among  the  most  singular  of  these 
are  several  species  of  Ichthyosaurus  and  Plesiosaurus.  The 
genus  Ichthyosaurus,  or  fish-lizard,  is  not  confined  to  this  form- 
ation, but  has  been  found  in  strata  as  high  as  the  chalk-marl  and 
gault  of  England,  and  as  low  as  the  muschelkalk,  a  formation 
which  immediately  succeeds  the  lias  in  the  descending  order.f 
It  is  evident  from  their  fish-like  vertebrae,  their  paddles,  resem- 
bling those  of  a  porpoise  or  whale,  the  length  of  their  tail,  and 
other  parts  of  their  structure,  that  the  habits  of  the  Ichthyosaurs 
were  aquatic.  Their  jaws  and  teeth  show  that  they  were  carni- 
vorous ;  and  the  half-digested  remains  of  fishes  and  reptiles, 
found  within  their  skeletons,  indicate  the  precise  nature  of  their 
food4  Mr.  Conybeare  was  enabled,  in  1824,  after  examining 
many  skeletons  nearly  perfect,  to  give  an  ideal  restoration  of 
the  osteology  of  this  genus,  and  of  that  of  the  Plesiosaurus. § 
The  latter  animal  had  an  extremely  long  neck  and  small  head, 
with  teeth  like  those  of  the  crocodile,  and  paddles  analogous  to 
those  of  the  Ichthyosaurus,  but  larger.  It  is  supposed  to  have 
lived  in  shallow  seas  and  estuaries,  and  to  have  breathed  air  like 
the  Ichthyosaur,  and  our  modern  cetacea.||  Some  of  the  rep- 
tiles above  mentioned  were  of  formidable  dimensions.  One 
specimen  of  Ichthyosaurus  playtyodon,  from  the  lias  at  Lyme, 
now  in  the  British  Museum,  must  have  belonged  to  an  animal 
more  than  twenty-four  feet  in  length,  and  another  of  the  Plesio- 
saurus, in  the  same  collection,  is  eleven  feet  long.  The  form  of 
the  Ichthyosaurus  may  have  fitted  it  to  cut  through  the  waves 
like  the  porpoise ;  but  it  is  supposed  that  the  Plesiosaurus,  at 
least  the  long-necked  species  (Fig.  228.),  was  better  suited  to 
fish  in  shallow  creeks  and  bays,  defended  from  heavy  breakers. 

*  Bridgewater  Treatise,  p.  290.  t  Buckland,  Bridgew.  Treat.,  p.  168. 

t  Ibid.  p.  187.  §  Geol.  Trans.,  Second  Series,  vol.  i.  pi.  49. 

II  Conybeare  and  De  la  Beche,  Geol.  Trans.;  and  Buckland,  Bridgew. 
Treat.,  p.  203. 


238 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Reptiles  of  the  Lias. 


•a   .SP 


For  the  last  twenty  years,  anatomists  have  agreed  that  these 
extinct  saurians  must  have  inhabited  the  sea,  although  no  living 
marine  reptile  was  known.  They  argued  that,  as  there  are  now 
Chelonians,  like  the  tortoise,  living  in  fresh  water,  and  others, 
as  the  turtle,  frequenting  the  ocean,  so  there  may  have  been 


PART  II.    CHAPTER  XVIII.  229 


Living  Marine  Saurian  of  the  Galapagos  Islands. 


formerly  some  saurians  proper  to  salt,  others  to  fresh  water. 
The  recent  discovery,  however,  of  a  maritime  saurian,  has  now 
rendered  it  unnecessary  to  speculate  on  such  possibilities.  This 
creature  was  found  in  the  Galapagos  islands,  during  the  visit  of 
H.  M.  S.  Beagle  to  that  archipelago,  in  1835,  and  its  habits  were 
then  observed  by  Mr.  Darwin.  The  islands  alluded  to  are  situ- 
ated under  the  equator,  nearly  600  miles  to  the  westward  of  the 
coast  of  South  America.  They  are  volcanic,  some  of  them 
being  3000  or  4000  feet  high ;  and  one  of  them,  Albemarle 
Island,  75  miles  long.  The  climate  is  mild,  very  little  rain  falls  ; 
and,  in  the  whole  archipelago,  there  is  only  one  rill  of  fresh 
water  that  reaches  the  coast.  The  soil  is  for  the  most  part  dry 
and  harsh,  and  the  vegetation  scanty.  The  birds,  reptiles,  plants, 
and  insects  are,  with  very  few  exceptions,  of  species  found  no- 
where else  in  the  world,  although  all  partake,  in  their  general 
form,  of  an  American  character.  Of  the  mammalia,  says  Mr. 
Darwin,  one  species  alone  appears  to  be  indigenous,  namely,  a 
large  and  peculiar  kind  of  mouse ;  but  the  number  of  lizards, 
tortoises,  and  snakes  is  so  great  that  it  may  be  called  a  land  of 
reptiles.  The  variety,  indeed,  of  species  is  small ;  but  the  indi- 
viduals of  each  are  in  wonderful  abundance.  There  is  a  turtle, 
a  large  tortoise  (Testudo  Indicus),  four  lizards,  and  about  the 
same  number  of  snakes,  but  no  frogs  or  toads.  Two  of  the 
lizards  belong  to  the  family  Iguanidce  of  Bell,  and  to  a  peculiar 
genus  (Amblyrhynchus)  established  by  that  naturalist ;  and  so 

Pig.  229. 


Amblyrhynchus  cristatus,  Bell,    Length  varying  from  3  to  4ft.     The  only 

existing  marine  lizard  now  known. 
a  Tooth  of  same  of  natural  size,  and  magnified. 

named  from  their  obtusely  truncated  head  and  short  snout.*     Of 
these  lizards,  one  is  terrestrial  in  its  habits,  and  burrows  in  the 

*  apBXvs,  amblys,  blunt,  and  py^o?,  rhynchus,  snout. 


230      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Living  Marine  Saurian. 

ground,  swarming  everywhere  on  the  land,  having  a  round  tail, 
and  a  mouth  somewhat  resembling  in  form  that  of  the  tortoise. 
The  other  is  aquatic,  and  has  its  tail  flattened  laterally  for  swim- 
ming (see  Fig.  229.).  "  This  marine  saurian,"  says  Mr.  Darwin, 
"  is  extremely  common  on  all  the  islands  throughout  the  archi- 
pelago. It  lives  exclusively  on  the  rocky  sea-beaches,  and  I 
never  saw  one  even  ten  yards  inshore.  The  usual  length  is 
about  a  yard,  but  there  are  some  even  four  feet  long.  It  is  of  a 
dirty  black  colour,  sluggish  in  its  movements  on  the  land;  but, 
when  in  the  water,  it  swims  with  perfect  ease  and  quickness  by 
a  serpentine  movement  of  its  body  and  flattened  tail,  the  legs 
during  this  time  being  motionless,  and  closely  collapsed  on  its 
sides.  Their  limbs  and  strong  claws  are  admirably  adapted  for 
crawling  over  the  rugged  and  fissured  masses  of  lava  which 
everywhere  form  the  coast.  In  such  situations  a  group  of  six 
or  seven  of  these  hideous  reptiles  may  oftentimes  be  seen  on  the 
black  rocks,  a  few  feet  above  the  surf,  basking  in  the  sun  with 
outstretched  legs.  Their  stomachs,  on  being  opened,  were  found 
to  be  largely  distended  with  minced  sea- weed,  of  a  kind  which 
grows  at  the  bottom  of  the  sea,  at  some  little  distance  from  the 
coast.  To  obtain  this,  the  lizards  are  seen  occasionally  going 
out  to  sea  in  shoals.  One  of  these  animals  was  sunk  in  salt 
water,  from  the  ship,  with  a  heavy  weight  attached  to  it,  and 
drawn  up  again  after  an  hour ;  it  was  quite  active  and  unharmed. 
It  is  not  yet  known  by  the  inhabitants  where  this  animal  lays 
its  eggs ;  a  singular  fact,  considering  its  abundance,  and  that 
the  natives  are  well  acquainted  with  the  eggs  of  the  terrestrial 
Amblyrhynchvs,  which  last  is  also  herbivorous,  although  feed- 
ing on  a  very  different  kind  of  vegetation."* 

In  those  deposits  now  forming  by  the  sediment  washed  away 
from  the  wasting  shores  of  the  Galapagos  islands,  the  remains 
of  saurians,  both  of  the  land  and  sea,  as  well  as  of  chelonians 
and  fish,  may  be  mingled  with  marine  shells  without  any  bones 
of  land  quadrupeds  or  batrachian  reptiles ;  yet  even  here  we 
should  expect  the  remains  of  marine  mammalia  to  be  imbedded 
in  the  new  strata,  for  there  are  seals,  besides  several  kinds  of 
cetacea,  on  the  Galapagian  shores;  and,  in  this  respect,  the 
parallel  between  the  modern  fauna,  above  described,  and  the 
ancient  one  of  the  lias,  would  not  hold  good. 

Sudden  destruction  of  saurians,  fyc. — It  has  been  remarked, 
and  truly,  that  many  of  the  fish  and  saurians,  found  fossil  in  the 
lias,  must  have  met  with  sudden  death  and  immediate  burial ;  and 

*  Danvin's  Journal,  chap.  xix. 


PART  II.     CHAPTER  XVIII.  231 


Sudden  Destruction  of  Marine  Animals. 


.hat  the  destructive  operation,  whatever  may  have  been  its  nature, 
was  often  repeated. 

"  Sometimes,"  says  Dr.  Buckland,  "  scarcely  a  single  bone  or 
scale  has  been  removed  from  the  place  it  occupied  during  life ; 
which  could  not  have  happened  had  the  uncovered  bodies  of  these 
saurians  been  left,  even  for  a  few  hours,  exposed  to  putrefaction, 
and  to  the  attacks  of  fishes  and  other  smaller  animals  at  the 
bottom  of  the  sea."*  Not  only  are  the  skeletons  of  the  Ichthyo- 
sauri entire,  but  sometimes  the  contents  of  their  stomachs  still 
remain  between  their  ribs,  so  that  we  can  discover  the  particular 
species  offish  on  which  they  lived,  and  the  form  of  their  excre- 
ments. Not  unfrequently  there  are  layers  of  these  coprolites  at 
different  depths  in  the  lias,  at  a  distance  from  any  entire  skele- 
tons of  the  marine  lizards,  from  which  they  were  derived,  "  as 
if,"  says  M.  De  la  Beche,  "  the  muddy  bottom  of  the  sea  received 
small  sudden  accessions  of  matter  from  time  to  time,  covering 
up  the  coprolites  and  other  exuviae  which  had  accumulated  during 
the  intervals."!  It  is  further  stated  that,  at  Lyme  Regis,  those 
surfaces  only  of  the  coprolites  which  lay  uppermost  at  the  bottom 
of  the  sea  have  suffered  partial  decay,  from  the  action  of  water 
before  they  were  covered  and  protected  by  the  muddy  sediment 
that  has  afterwards  permanently  enveloped  them.ij; 

Numerous  specimens  of  the  pen-and-ink  fish  (Sepia  loligo, 
Lin.,  Loligo  vulgaris,  Lam.)  have  also  been  met  with  in  the 
lias  at  Lyme,  with  the  ink-bags  still  distended,  containing  the 
ink  in  a  dried  state,  chiefly  composed  of  carbon,  and  but  slightly 
impregnated  with  carbonate  of  lime.  These  cephalopoda,  there- 
fore, must,  like  the  saurians,  have  died  suddenly,  and  have  been 
instantly  buried  in  sediment;  for,  if  exposed  after  death,  the 
membrane  containing  the  ink  would  have  decay  ed.§ 

As  we  know  that  river  fish  are  sometimes  stifled,  even  in  their 
own  element,  by  muddy  water  during  floods,  it  cannot  be  doubt- 
ed that  the  periodical  discharge  of  large  bodies  of  turbid  fresh 
water  into  the  sea  may  be  still  more  fatal  to  marine  tribes.  In 
the  Principles  of  Geology,  I  have  shown  how  large  quantities  of 
mud  and  drowned  animals  are  swept  down  into  the  sea  by  rivers 
during  earthquakes,  as  in  Java,  in  1699;  and  how  undescriba- 
ble  multitudes  of  dead  fish  have  been  seen  floating  on  the  sea 
after  a  discharge  of  noxious  vapours  from  similar  convulsions. || 
But,  in  the  intervals  between  such  catastrophes,  strata  may  have 

*  Bridgew.  Treat.,  p.  125.  t  Geological  Researches,  p.  334. 

t  Buckland,  Bridgew.  Treat.,  p.  307. "  $  Ibid. 

||  See  Principles  of  Geology,  Index,  "  Lancerote,"  "Graham  Island,"  "Cala- 
bria," 


232      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Fossils  of  the  Lias Origin  of  the  Oolite  and  Lias. 

accumulated  slowly  in  the  sea  of  the  lias,  some  being  formed 
chiefly  of  one  description  of  shell,  such  as  ammonites,  others  of 
gryphites. 

Fossil  plants. — Among  the  vegetable  remains  of  the  Lias, 
several  species  of  Zamia  have  been 
Fig.  230.  found  at  Lyme  Regis,  and  the  re- 

mains of  coniferous  plants  at  Whitby. 
Fragments  of  wood  are  common,  and 
often  converted  into  argillaceous  lime- 
stone. That  some  of  this  wood, 
though  now  petrified,  was  soft,  when 
it  first  lay  at  the  bottom  of  the  sea, 
is  shown  by  a  specimen  now  in  the  museum  of  the  Geological 
Society,  (see  Fig.  230.)  which  has  the  form  of  an  ammonite 
indented  on  its  surface. 

Origin  of  the  Oolite  and  Lias.  —  If  we  now  endeavour  to 
restore,  in  imagination,  the  ancient  condition  of  the  European 
area  at  the  period  of  the  Oolite  and  Lias,  we  must  conceive  a 
sea  in  which  the  growth  of  coral  reefs  and  shelly  limestones, 
after  proceeding  without  interruption  for  ages,  was  liable  to  be 
stopped  suddenly  by  the  deposition  of  clayey  sediment.  Then, 
again,  the  argillaceous  matter,  devoid  of  corals,  was  deposited 
for  ages,  and  attained  a  thickness  of  hundreds  of  feet,  until  ano- 
ther period  arrived  when  the  same  space  was  again  occupied  by 
calcareous  sand,  or  solid  rocks  of  shell  and  coral,  to  be  again 
succeeded  by  the  recurrence  of  another  period  of  argillaceous 
deposition.  Mr.  Conybeare  has  remarked  of  the  entire  group  of 
Oolite  and  Lias,  that  it  consists  of  repeated  alternations  of  clay, 
sandstone,  and  limestone,  following  each  other  in  the  same  order. 
Thus  the  clays  of  the  lias  are  followed  by  the  sands  of  the  infe- 
rior oolite,  and  these  again  by  shelly  and  coralline  limestone, 
(Bath  oolite,  &c.;)  so,  in  the  middle  oolite,  the  Oxford  clay  is 
followed  by  calcareous  grit  and  "  coral  rag ;"  lastly,  in  the 
upper  oolite  the  Kimmeridge  clay  is  followed  by  the  Wey mouth 
sands  and  the  Portland  limestone.*  The  clay  beds,  however,  as 
Mr.  De  la  Beche  remarks,  can  be  followed  over  larger  areas 
than  the  sands  or  sandstones. f  It  should  also  be  remembered, 
that  while  the  oolitic  system  becomes  arenaceous,  and  resembles 
a  coal-field  in  Yorkshire,  it  assumes,  in  the  Alps,  an  almost 
purely  calcareous  form,  the  sands  and  clays  being  omitted  ;  and 
even  in  the  intervening  tracts,  it  is  more  complicated  and  vari- 
able than  appears  in  ordinary  descriptions.  Nevertheless,  some 
of  the  clays  and  intervening  limestones  do,  in  reality,  retain  a 

*  Con.  and  Phil.  p.  166.  t  Geol.  Researches,  p.  337. 


PART  II.     CHAPTER  XVIil.  233 

Origin  of  the  Oolite  and  Lias. 

pretty  uniform  character,  for  distances  of  from  400  to  600  miles 
from  east  to  west  and  north  to  south. 

According  to  M.  Thirria,  the  entire  oolitic  group  in  the  depart- 
ment of  the  Haute  Saone,  in  France,  may  be  equal  in  thickness 
to  that  of  England ;  but  the  importance  of  the  argillaceous  divi- 
sions is  in  the  inverse  ratio  to  that  which  they  exhibit  in  Eng- 
land, where  they  are  about  equal  to  twice  the  thickness  of  the 
limestones,  whereas,  in  the  part  of  France  alluded  to,  they  reach 
only  about  a  third  of  that  thickness.*  In  the  Jura  the  clays  are 
still  thinner  ;  and  in  the  Alps  they  thin  out  and  almost  vanish. 

In  order  to  account  for  such  a  succession  of  events,  we  may 
imagine,  first,  the  bed  of  the  ocean  to  be  the  receptacle  for  ages 
of  fine  argillaceous  sediment,  brought  by  oceanic  currents, 
which  may  have  communicated  with  rivers,  or  with  part  of  the 
sea  near  a  wasting  coast.  This  mud  ceases,  at  length,  to  be 
conveyed  to  the  same  region,  either  because  the  land  which  had 
previously  suffered  denudation  is  depressed  and  submerged,  or 
because  the  current  is  deflected  in  another  direction  by  the 
altered  shape  of  the  bed  of  the  ocean  and  neighbouring  dry 
land.  By  such  changes  the  water  becomes  once  more  clear  and 
fit  for  the  growth  of  stony  zoophytes.  Calcareous  sand  is  then 
formed  from  comminuted  shell  and  coral,  or,  in  some  cases,  are- 
naceous matter  replaces  the  clay,  because  it  commonly  happens 
that  the  finer  sediment,  being  first  drifted  farthest  from  coasts,  is 
subsequently  overspread  by  coarse  sand,  after  the  sea  has  grown 
shallower,  or  when  the  land,  increasing  in  extent,  has  approached 
nearer  to  the  spots  first  occupied  by  fine  mud. 

In  order  to  account  for  another  great  formation,  like  the  Ox- 
ford clay,  again  covering  one  of  coral  limestone,  we  must  sup- 
pose a  sinking  down  like  that  which  is  now  taking  place  in  some 
existing  regions  of  coral  between  Australia  and  South  America. f 
*  The  occurrence  of  subsidences,  on  so  vast  a  scale,  may  again 
have  caused  the  bed  of  the  ocean  and  the  adjoining  land  through- 
out the  European  area,  to  assume  a  shape  favourable  to  the 
deposition  of  another  set  of  clayey^strata ;  ~~and  this  change  may 
have  been  succeeded  by  a  series  of  events  analogous  to  that 
already  explained,  and  these  again  by  a  third  series  in  similar 
order.  Both  the  ascending  and  descending  movements  may 
have  been  extremely  slow,  like  those  now  going  on  in  the 
Pacific ;  and  the  growth  of  every  stratum  of  coral,  a  few  feet 
in  thickness,  may  have  required  centuries  for  its  completion, 
during  which  certain  species  of  organic  beings  may  have  disap- 
peared from  the  earth,  and  others  have  been  introduced  in  their 


*  Burat's  D'Aubuisson,  torn.  ii.  p.  456.  t  See  Darwin,  chap.  xxii. 


234  LYELL'S  ELEMENTS  OF  GEOLOGY. 


Valleys  and  Escarpments  of  Chalk,  Oolite,  and  Lias. 

place ;  so  that,  in  each  set  of  strata,  from  the  Upper  Oolite  to 
the  Lias,  some  peculiar  and  characteristic  fossils  were  imbedded. 

Physical  geography. — The  alternation,  on  so  large  a  scale, 
of  distinct  formations  of  clay  and  limestone,  has  given  rise  to 
some  marked  features  in  the  physical  outline  of  parts  of  Eng- 
land and  France.  Wide  valleys  can  usually  be  traced  through- 
out the  long  bands  of  country  where  the  argillaceous  strata 
crop-out ;  and  between  these  valleys  the  limestones  are  observed, 
composing  ranges  of  hills,  or  more  elevated  grounds*  These 
ranges  terminate  abruptly  on  the  side  on  which  the  several  clays 
crop-out  from  beneath  the  calcareous  strata. 

The  annexed  diagram  will  give  the  reader  an  idea  of  the  con- 
figuration of  the  surface  now  alluded  to,  such  as  may  be  seen  in 
passing  from  London  to  Cheltenham,  or  in  other  parallel  lines, 
from  east  to  west,  in  the  southern  part  of  England.  It  has  been 

Fig.  231. 

Middle  Upper  London 

Oolite.  Oolite.         Chalk,     clay. 


Lias.  Oxford  clay.    .  Kim.  clay.          Gault. 

necessary,  however,  in  this  drawing,  greatly  to  exaggerate  the 
inclination  of  the  beds,  and  the  height  of  the  several  formations, 
as  compared  to  their  horizontal  extent.  It  will  be  remarked, 
that  the  lines  of  cliff,  or  escarpment,  face  towards  the  west  in 
the  great  calcareous  eminences  formed  by  the  Chalk  and  the 
Upper,  Middle,  and  Lower  Oolites ;  and  at  the  base  of  each  we 
have  respectively  the  Gault,  Kimmeridge  clay,  Oxford  clay,  and 
Lias.  This  last  forms,  generally,  a  broad  veil  at  the  foot  of  the 
escarpment  of  Inferior  Oolite;  but  a  considerable  portion  of 
that  escarpment  is  sometimes  occupied  by  lias.  The  external 
outline  of  the  country  which  the  geologist  observes  in  travelling 
westward  from  Paris  to  Metz,  is  precisely  analogous,  and  is 
caused  by  a  similar  succession  of  rocks  intervening  between  the 
tertiary  strata  and  the  Lias ;  with  this  difference,  however,  that 
the  escarpments  of  Chalk,  Upper,  Middle,  and  Inferior  Oolites, 
face  towards  the  east  instead  of  the  west. 

The  Chalk  crops-out  from  beneath  the  tertiary  sands  and 
clays  of  the  Paris  basin,  near  Epernay,  and  the  Gault  from 
beneath  the  Chalk  and  Upper  Green-sand  at  Clermon*  en  Ar- 
gonne ;  and  passing  from  this  place  by  Verdun  and  Etain  to 
Metz,  we  find  two  limestone  ranges,  with  intervening  vales  of 
clay,  precisely  resembling  those  of  southern  and  central  Eng- 


PART  II.     CHAPTER  XIX.  235 

New  Red  Sandstone. 

land,  until  we  reach  the  great  plain  of  Lias  at  the  base  of  the 
Inferior  Oolite  at  Metz. 

It  is  evident,  therefore,  that  the  denuding  causes  have  acted 
similarly  over  an  area  several  hundred  miles  in  diameter,  sweep- 
ing away  the  softer  clays  more  extensively  than  the  limestones, 
and  undermining  these  last  so  as  to  cause  them  to  form  steep 
cliffs  wherever  the  harder  calcareous  rock  was  based  upon  a 
more  yielding  and  destructible  clay.  This  denudation  probably 
occurred  while  the  land  was  slowly  rising  out  of  the  sea.* 


CHAPTER  XIX. 


NEW   RED   SANDSTONE   GROUP. 

Distinction  between  New  and  Old  Red  sandstone — Between  Upper  and  Lower 
New  Red — Muschelkalk  in  Germany — Fossil  plants  and  shells  of  New  Red 
Group,  entirely  different  from  Lias  and  Magnesian  limestone — Lower  New  Red 
and  Magnesian  limestone — Zechstein  in  Germany  of  the  same  age — General 
resemblance  between  the  organic  remains  of  the  Magnesian  limestone  and  Car- 
boniferous strata— Origin  of  red  sandstone  and  red  marl. 

BETWEEN  the  Lias  and  the  Coal,  or  Carboniferous  group, 
there  is  interposed  in  the  midland  and  western  counties  of  Eng- 
land a  great  series  of  red  marls  and  sandstones,  to  which  the 
name  of  the  New  Red  Sandstone  formation  was  given,  to  dis- 
tinguish it  from  the  other  marls  and  sandstones  called  the  "  Old 
Red,"  (c.  Fig.  232.)  often  identical  in  mineral  character,  which 
lie  immediately  beneath  the  coal,  b. 

Fig.  232. 

Coal.  New  Eed  sandstone. 


In  some  parts  of  the  south-west  of  England,  the  entire  "  New 
Red"  group  consists  exclusively  of  red  loam,  clay,  and  sand- 
stone, devoid  of  fossils,  strongly  contrasted  in  colour,  and  the 
general  absence  of  calcareous  matter,  with  the  Oolitic  rocks  and 

*  See  Principles  of  Geology,  Index,  Weatden  denudation. 


236  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Upper  New  Red  Sandstone. 

Lias  before  described.  But  when  we  extend  our  observations 
over  England  and  other  countries,  we  no  longer  find  this  simpli- 
city of  structure ;  but  perceive  that  the  strata  between  the  Lias 
and  the  Coal  are  divisible  into  two  very  distinct  systems,  which 
will  be  understood  from  the  accompanying  Table,  and  the 
description  which  follows. 

NEW  RED  SANDSTONE  GROUP. 
Poikilitic  group  of  Conybeare  and  Buckland.* 

Synonyms. 


German.  French. 

'a.    Saliferous  marls 
and  sandstone  . . . 
b.  (wanting  in  Eng- 


land) 


c.  Sandstone  and 
quartzose  conglo- 
merate   


Keuper Marnes  irisees. 

A/r      v  11    n  (  Muschelkalk,  ou  cal- 

Muschelkalk  . . . .  J      caire  CO(lui\liere> 

Bunter  sandstein    >  Gres  bigarre. 


£       frf.  Magnesian  lime- ^  Zechstein,     and 
stone      (dolomiticS-     Kupfer   schie- 


conglomerate)  . . .  )     fer 


&  ®  i 

o  ^  \e.  Lower  New  Red  >  T,  .,  ,•     „„,,  B 

2  sandstone.  5  Roth-hegendes  . 


Zechstein,  ou  schiste 
cuivreux  —  et  Cal- 
caire  Magnesien. 

Gres  des  Vosges, 
couches  inferi- 
eures? 


UPPER  NEW  RED  SANDSTONE. 
(Including  the  Muschelkalk  of  the  Germans.) 

The  Lias  is  succeeded  in  England  by  strata  of  red  and  green 
marl,  or  clay,  which  are  conformable  to  the  Lias,  and  pass  into 
it,  as  in  Gloucestershire.  It  is  in  this  upper  New  Red  system 
that  rock-salt  and  salt  springs  occur  in  Cheshire  and  other  parts 
of  England  ;  and  to  this,  therefore,  the  term  "  Saliferous  marl 
and  sandstone  formation,  is  properly  applicable."f  It  consists, 
in  Cheshire,  of  alternating  beds  of  red  and  green  clay,  or  marl, 
gypsum,  and  rock-salt,  upwards  of  600  feet  in  thickness. 

A  few  traces  only  of  fossil  shells,  fish,  and  plants  have  been 
detected  in  this  formation  in  England  ;  but  in  a  corresponding 
position  in  Germany  there  occur  similar  strata  of  red  sandstone 
and  marl,  in  which  are  many  organic  remains,  and  associated 
with  the  same  a  great  calcareous  formation  called  the  "  Mus- 


*  From  TToiKiXos,  Poikilos,  variegated,  see  Buckland,  Bridgw.  Treat.,  vol.  ii. 
p.  38.,  because  some  of  the  most  characteristic  strata  of  this  group  were  called 
variegated  by  Werner,  from  their  exhibiting  spots  and  streaks  of  light  blue, 
green,  and  buff  colour,  in  a  red  base. 

t  Murchison,  Silurian  System,  p.  32. 


PART  II,     CHAPTER  XIX.  237 


Muschelkalk. 


chelkalk,"  or  "  shelly-limestone."  As  the  fossil  fauna  and  flora 
of  these  formations  supply  the  chasm  which  exists  in  our  British 
series,  I  shall  say  a  few  words  of  the  "  Upper  New  Red,"  as  it 
appears  in  Bavaria  and  Wurtemberg.  First  in  order  beneath  the 
Lias  come  mottled  marls  and  sandstones,  red,  green,  purple,  and 
white,  containing  gypsum  and  salt ;  then  the  Muschelkalk  above 
mentioned,  and  then  another  set  of  marls  and  sandstones  much 
resembling  the  first.  That  these  three  formations,  the  Keuper, 
Muschelkalk,  and  Bunter  Sandstein,  (see  Table,)  may  be 
referred  to  one  period,  appears  from  the  fact  that  Count  Munster 
has  obtained  the  same  plants  from  the  Keuper  and  Bunter  Sand- 
stein  ;  and  M.  Agassiz  the  same  species  of  fish  from  both  of 
them,  and  from  the  interposed  Muschelkalk.  It  is  also  worthy 
of  remark,  that  the  strata  of  the  Muschelkalk  alternate  with 
those  of  the  Keuper  and  Bunter  Sandstein  at  their  junction. 

The  fossil  Flora,  above  alluded  to,  consists  of  Cycadeas  and 
several  genera  of  ferns,  also  extinct  coniferre  of  the  genus 
Voltzia  (Ad.  Brongniart)  peculiar  to  this  period,  in  which  even 
the  fructification  has  been  preserved,  (Fig.  233.)  and  a  gigantic 
species  of  Equisetum,  (Fig.  234.)  which  is  not  uncommon  in  the 
Keuper  sandstone. 


Fig.  233.  Fig.  234. 


Voltzia  brevifolia,  and  portion  magnified  Equisetum  columnare ;  fragment 

to  show  fructification  ;   Sulzbad.  of  stem,  and  small  portion  of 

Keuper  and  Bunter  Sandstein.  same  magnified.    Keuper. 

X 

Among  the  shells,  some  of  the  Cephalopoda  are  peculiar,  as, 
for  example,  that  form  of  Ammonite  which  is  called  Ceratite  by 
De  Haan,  in  which  the  descending  lobes,  see  a,  b,  c,  Fig.  235., 
terminate  in  a  few  small  denticulations  pointing  inwards.  Among 
the  bivalve  shells,  the  Posidonia  keuperina,  Voltz.  (Posidonomya 
minuta,  Bronn,  Fig.  236.)  is  abundant,  ranging  from  the  Keuper 
to  the  Bunter  Sandstein ;  and  the  Avicula  socialis,  (Fig.  237.) 
having  a  similar  range,  but  most  characteristic  of  the  Muschel- 
kalk in  Germany,  France,  and  Poland. 

There  are  also  some  encrinites  in  the  Muschelkalk,  and  some 
teeth  of  cartilaginous  fish,  a  few  decapod  Crustacea,  and  no  less 


238      L  YELL'S  ELEMENTS  OF  GEOLOGY. 


Fossil  Footsteps. 

Fig.  235. 


Ammonites  nodosus.    Muschelkalk. 

a.  Side  view.  b.  Front  view. 

c.  Partially  denticulated  outline  of  the  septa  dividing  the  chambers. 


Fig.  236. 


Fig.  237. 


Posidonomya 
ininuta,  Bronn. 


a.  dvicula  socialis.  b.  Side  view  of  same. 

Characteristic  of  the  Muschelkalk. 


than  five  genera  of  large  extinct  reptiles,  all  peculiar  to  the 
Muschelkalk,  as  Phytosaurus,  Dracosaurus,  and  others.  Upon 
the  whole,  Professor  Bronn  has  enumerated,  in  his  LethaBa 
Geognestica,  no  less  than  forty-seven  genera  of  fossil  remains 
from  the  three  divisions  of  the  "  Upper  New  Red"  system  in 
Germany ;  and  these  fossils  are  the  more  important  as  being  all 
distinct  in  species,  and  many  of  them  in  genera,  from  those  of 
the  incumbent  Lias  or  more  ancient  Magnesian  limestone. 

In  the  Bunter  Sandstein  near  Hildburghausen,  some  remark- 
able fossil  footsteps  have  lately  been 
discovered  in  quarries  of  a  gray  quart- 
zose  sandstone.  On  the  upper  surfaces 
of  the  slabs  of  stone  the  steps  form  de- 
pressions, while  those  on  the  lower  sur- 
faces are  in  relief.  These  last  are 
natural  casts  formed  in  the  subjacent 
footsteps,  as  in  moulds.  The  larger 
prints  seem  to  be  those  of  the  hind  foot, 
and  are  about  eight  inches  long  and  five 
wide.  Near  to  each,  and  at  the  regular 
distance  of  an  inch  and  a  half  before 
is  a  smaller  print  of  a  fore-foot 


Fig.  238. 


Single  footstep  of  Chirotherium.       CUStan 
one  eighth  of  nat.  size.  them 


PART  II.     CHAPTER  XIX.  239 

Lower  New  Red  Sandstone. 

(see  Fig.  238.).  In  each  pair  of  large  and  small  steps,  the 
great  toes  are  turned  alternately  both  to  the  right  or  both  to  the 
left.* 

For  this  unknown  animal,  Professor  Kaup  has  proposed  the 
provisional  name  of  Chirotherium  ;  and  he  conjectures  that  it  was 
a  mammiferous  quadruped,  allied  to  the  marsupialia.f 

Fig.  239. 


Line  of  footsteps  on  slab  of  sandstone.    Hildburghausen,  in  Saxony. 


In  the  kangaroo,  says  Dr.  Buck  land,  the  first  toe  of  the  fore- 
foot is,  in  a  similar  manner,  set  obliquely  to  the  others,  like  a 
thumb ;  and  the  disproportion  between  the  fore  and  hind-feet  is 
also  very  great.  If  it  should  be  eventually  proved  that  this  ani- 
mal was  really  marsupial,  these  fossil  relics  belong  to  the  most 
ancient  mammiferous  quadruped  yet  known  to  palaeontologists. 

It  would  scarcely  be  possible  to  draw  a  distinct  line  of  de- 
marcation between  the  Keuper  and  Bunter  Sandstein,  in  Germa- 
ny, where  they  are  not  barren  of  fossils,  if  the  Muschelkalk  did 
not  intervene  between  them.  In  England,  therefore,  where  this 
calcareous  formation  is  wanting,  and  where  there  are  scarcely 
any  organic  remains  in  the  Upper  New  Red  marl  and  sandstone, 
we  cannot  feel  assured  that  the  divisions  a.  and  c.  of  our  Table, 
p.  236.,  do  really  coincide  with  the  German  Keuper  and  Bunter 
Sandstein.  But  it  has  been  found  convenient  in  the  counties  of 
Salop,  Stafford,  and  Worcester,  to  divide  the  saliferous  marls 
from  the  inferior  quartzose  conglomerate  in  the  manner  above 
indicated. 

LOWER  NEW  RED  SANDSTONE  AND  MAGNESIAN  LIMESTONE. 

We  now  come  to  the  Lower  New  Red  system,  the  position  of 
which  can  best  be  determined  in  Germany,  because  it  is  there 
interposed  between  the  Coal  and  Bunter  Sandstein,  or  oldest  part 
of  the  "  Upper  New  Red,"  above  described.  In  the  south-west 
of  England  the  New  Red  sandstone  formation  is  unconformable 
to  the  Coal  (see  Fig.  232.) ;  but  in  the  north-east  of  England 
Professor  Sedgwick  has  shown  that  the  same  series  is  conform- 
able to  the  carboniferous  strata,  and  passes  into  them.  In  other 
words,  the  movements  which  deranged  "  the  Coal"  in  the 

*  One  of  these  slabs  is  now  in  the  British  Museum, 
t  See  Buckland's  Bridgew.,  p.  2C3. 


210  LYELL'S  ELEMENTS  OF  GEOLOGY. 


Lower  New  Red  and  Magnesian  Limestone.     * 


south-west,  previously  to  the  origin  of  the  New  Red  sandstone, 
did  not  extend  towards  Durham  and  the  more  northern  counties. 

Near  Bristol,  in  Somersetshire,  and  in  other  counties  border- 
ing the  Severn,  the  unconformable  beds  of  the  Lower  New  Red, 
resting  immediately  upon  the  Coal,  consist  of  a  conglomerate 
called  "  dolomitic,"  because  the  pebbles  of  older  rocks  are  ce- 
mented together  by  a  base  of  magnesian  limestone.  Among  the 
imbedded  pebbles  are  many  derived  from  the  Coal,  particularly 
from  the  carboniferous  limestone,  the  peculiar  fossils  of  which 
are  still  seen  in  many  large  rounded  fragments.  In  the  north- 
east of  England  the  dolomitic  conglomerate  is  represented  by  a 
yellow  limestone,  generally  called  the  Magnesian  Limestone ; 
which  passes  upwards  into  marl  slate,  and  downwards  into  red 
marl  and  gypsum.  In  the  intermediate  counties  of  Worcester- 
shire, Staffordshire,  and  Shropshire,  are  conglomerates  referred 
to  the  same  age,  but  which  are  calcareous,  with  scarcely  any 
magnesia.  Between  these  conglomerates  and  the  Coal  is  a  great 
formation,  called  the  Lower  New  Red  sandstone  (see  Table,  p. 
236.),  composed  of  sandstones,  red  shales,  and  marls,  occasion- 
ally spotted  green.* 

The  country  of  Mansfeld,  in  Thuringia,  may  be  called  the 
classic  ground  of  the  Lower  New  Red,  or  Magnesian  Lime- 
stone formation  on  the  continent.  It  has  there  been  long  cele- 
brated, because  one  of  its  members,  a  slaty  marlstone,  is  richly 
impregnated  with  copper  pyrites,  for  which  it  is  extensively 
worked.  The  formation  in  that  country  is  composed  of  an  upper 
calcareous  division,  called  the  Zechstein,  and  a  lower  red  quart- 
zose  formation  of  sandstone  and  conglomerate,  called  the  Roth- 
liegendes.  The  upper  of  these  systems  is  very  complex, 
consisting  of  marl,  limestone,  copper-slate,  magnesian  limestone, 
gypsum,  and  rock-salt,  in  which  numerous  fossils  occur,  bearing 
a  striking  generic  resemblance  to  those  of  our  English  Magne- 
sian Limestone.  The  Lower  system,  or  Rothliegendes,  is  inter- 
posed between  the  Zechstein  and  the  Coal ;  and  is  supposed  to 
correspond  with  the  Lower  New  Red  sandstone,  above  mentioned, 
as  occupying  a  similar  place  in  England  between  our  Magnesian 
Limestone  and  Coal.  Its  local  name  of  Rothliegendes,  red-Iyer, 
or  "  Roth-todt-liegendes,"  red-dead-lyer,  was  given  by  the  work- 
men in  the  German  mines  from  its  red  colour,  and  because  the 
copper  has  died  out  when  they  reach  this  rock  which  is  not 
metalliferous.  It  is,  in  fact,  a  great  deposit  of  red  sandstone 
and  conglomerate,  with  associated  porphyry,  basaltic  trap,  and 
amygdaloid. 

*  Murchison,  Silurian  System,  p.  54. 


PART  II.     CHAPTER  XIX. 


241 


Fossils  of  the  Magnesian  Limestone. 


When  we  consider  the  fossils  of  the  Magnesian  Limestone  in 
England,  or  corresponding  Zechstein  in  Germany,  we  find  that 
they  approach  much  nearer  in  their  character  to  the  organic 
remains  of  the  older  carboniferous  group  than  to  those  of  the 
Upper  New  Red.  Thus,  for  example,  the  two  genera  of  shells, 
Producta  and  Spirifer,  of  the  family  Brachiopoda,  are  common 


Fig.  240. 


Fig.  241. 


Magnesian  Limestone. 


Spirifer  undulatus,  Sow. 
Magnesian  Limestone. 


to  the  Magnesian  Limestone,  Coal,  and  Primary  fossiliferous 
strata,  but  have  never  been  met  with  in  any  rock  above  the 
Magnesian  Limestone.  There  are  certain  fish  also  found  both 
in  England  and  Germany,  in  the  Lower  New  Red  System, 
which  occur  in  the  carboniferous  strata,  but  in  no  formation 
higher  in  the  series  than  the  Magnesian  limestone,  not  even  in 
the  Muschelkalk. 

The  genus  Palaoniscus,  Agas.  (Pal&othrissum,  Blain.)  is 

Fig.  242. 


Restored  outline  of  a  fish  of  the  genus  Paleeoniscus.    Agass.*    Magnesian 
Limestone. 

the  most  striking  example,  as  three  species  have  been  found  in 
England  in  marl  slate,  immediately  below  the  Magnesian  Lime- 
stone ;  and  three  other  different,  but  nearly  allied  species,  in  the 
slate  of  the  Zechstein  of  Germany,  f 

It  was  first  pointed  out  by  M.  Agassiz,  that  all  the  bony  fish 
of  the  Magnesian  Limestone,  and  of  all  the  more  ancient  form- 
ations, have  the  vertebral  column  continued  into  the  upper  lobe 


*  Poissons  Fossiles,  vol.  i.  tab.  A.  fig.  4. 

t  Sedgwick,  Geol.  Trans.,  Second  Series,  vol.  iii.  p.  117. 


242  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Origin  of  Red  Sandstone  and  Marl. 

of  the  tail,  which  is  much  longer  than  the  lower  lobe  (see  Fig. 
242.),  whereas,  in  strata  newer  than  the  Magnesian  Limestone, 
the  tail-fin  is  divided  into  two  equal  lobes,  as  in  almost  all  living 
fishes,  the  vertebrae  not  being  prolonged  into  either  lobe. 

The  remains  of  at  least  two  saurian  animals  of  new  genera, 
Paloeosaurus  and  Thecodontosaurus  have  been  lately  discovered 
in  the  dolomitic  conglomerate  near  Bristol.*  They  are  allied  to 
the  Iguana  and  Monitor,  and  are  the  most  ancient  examples  of 
fossil  reptiles  yet  found  in  Great  Britain.  The  Zechstein  of 
Germany  is  also  the  oldest  rock  on  the  continent  in  which  Sau- 
rian remains  have  been  found.  They  are  referred  to  a  genus 
called  Protorosaurus,  also  allied  to  the  Monitor. 

The  resemblance  above  alluded  to  between  the 
Fig.  243.        fossils  of  the  Lower  New  Red  system  and  those 
iHHi 'iffifli    °f  tne  Coal,  is  not  confined  to  the  mollusca,  fish, 
and  reptiles,    but  extends   to  the  Crinoidea,  or 
Stone-lilies.     Thus  one  species,  the  Cyathocri- 
nites  planus  (Fig.  243.)  of  the  Magnesian  Lime- 
stone  of  Durham,  has   been  identified   by   Mr. 
Miller  with  a  fossil  of  the  Mountain  limestone  of 
Bristol.t 

Origin  of  the  New  Red  Sandstone  group. — 
The  red  sandstone  and  red  marl,  which,  in  point 
of  thickness,    form  the   most   considerable   part 
both  of  the  upper  and  lower  New  Red  formation 
Cyathocrinites     in  England  and  Germany,  may  have  arisen  in 
MagiSiSi^Sd     great   Part    fr°m   tne   disintegration   of  various 
Mountain  Lime-   crystalline,  or  metamorphic  schists ;  and   some- 
times, as   in   parts  of  Saxony  and  Devonshire, 
from  porphyritic  trap  rocks  containing  much  oxide  of  iron.     In 
some  districts  of  the  eastern  Grampians  in  Scotland,  as  in  the 
north  of  Forfarshire,  the  sides  of  mountains  composed  of  gneiss, 
mica-schist,  and  clay-slate,  are  covered  with  alluvium,  derived 
from  the  disintegration  of  those  rocks ;  and  the  mass  of  detritus 
is  stained  by  oxide  of  iron,  of  precisely  the  same  colour  as  the 
Old  Red  sandstone  of  the  adjoining  Lowlands.     Now  this  allu- 
vium merely  requires  to  be  swept  down  to  the  sea,  or  into  a  lake, 
to  form  strata  of  red  sandstone  and  red  marl,  similar  to  those  of 
the  "  Old  Red"  or  New  Red  system,  or  those  of  the  cretaceous 
era  in  Spain  (see  p.  199.),  or  those  of  tertiary  origin,  as  at 
Coudes  and  Champheix,  in  Auvergne,  all  of  which  are  in  litho- 
logical  characters  quite  undistinguishable  from  one  another.  The 

*  See  paper  by  Messrs.  Riley  and  Stuchbury,  Proceedings  Geol.  Soc.  No.  46. 
t  Sedgwick,  Geol.  Trans.,  Second  Series,  vol.  iii.  p.  120. 


PART  II.     CHAPTER  XX.  243 

Carboniferous  Group. 

pebbles  of  gneiss  in  the  tertiary  red  sandstone  of  Auvergne, 
point  clearly  to  the  rocks  from  which  it  -has  been  derived.  The 
red  colouring  matter  may  have  been  furnished  by  the  decomposi- 
tion of  hornblende,  or  mica,  which  contain  oxide  of  iron  in  large 
quantity  (see  p.  102.). 

It  is  a  general  fact,  and  one  not  yet  accounted  for,  that  scarce- 
ly any  fossil  remains  are  preserved  in  stratified  rocks  in  which 
this  oxide  of  iron  abounds  ;  and  when  we  find  fossils  in  the  New 
or  Old  Red  sandstone  in  England,  it  is  in  the  grey  and  usually 
calcareous  beds,  that  they  occur. 


CHAPTER  XX. 

THE  COAL,  OR  CARBONIFEROUS  GROUP. 

Carboniferous  strata  in  the  south-west  of  England — Superposition  of  Coal- 
measures  to  Mountain  limestone — Departure  from  this  type  in  north  of  England 
and  Scotland — Freshwater  strata — Intermixture  of  freshwater  and  marine  beds 
— Sauroidal  fish — Fossil  plants — Ferns  and  Sigillarise — Lepidodendra — Calamites 
— Coniferse — Stigmariae. 

THE  next  group  which  we  meet  with  in  the  descending  order 
is  the  Carboniferous,  commonly  called  "The  Coal,"  because* 
many  beds  of  that  mineral,  in  a  more  or  less  pure  state,  are  in- 
terstratified  with  sandstone,  shale,  and  limestone,  of  which  the 
bulk  of  the  formation  is  made  up.  The  combustible  coal  itself, 
even  in  Great  Britain  and  Belgium,  where  it  is  most  abundant, 
constitutes  but  a  small  proportion  of  the  whole  mass.  In  the 
north  of  England,  for  example,  the  thickness  of  the  coal-bearing 
strata  has  been  estimated  at  3000  feet,  while  the  various  coal- 
seams,  20  or  30  in  number,  do  not  exceed  60  feet.* 

In  the  south-west  of  England,  in  Somersetshire,  and  in  South 
Wales,  the  Carboniferous  series  consists  of, 

(  Strata  of  shale,  sandstone,  and  grit,  with  occasional 
1st.  Coal-measures,  <     seams  of  coal,  sometimes  exceeding  600  feet  in 
;'J;  "~  Q     thickness. 

f  A  coarse  quartzose  sandstone  passing  into  a  con- 
2d.  Millstone  grit.     <     glomerate,   sometimes  used  for   millstones;  de- 
(     void  of  coal ;  occasionally  above  600  feet  thick. 
3d.  Mountain  or       C  A   calcareous  rock   containing  marine  shells  and 
Carboniferous     <  corals,  devoid  of  coal ;  thickness  variable  ;  some- 
limestone,  r  times  900  feet. 


*  Phillips;  art.  "Geology,"  Encyc.  Britan. 


244  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Carboniferous  Group Freshwater  Strata. 

Beneath  all  these  is  the  Old  Red  sandstone,  which  was  for- 
merly considered  as  part  of  the  Carboniferous  series  ;  but  which, 
now  that  its  organic  remains  are  better  known,  appears  entitled 
to  rank  as  a  distinct  formation. 

As  we  proceed  northwards  from  South  Wales  and  Somerset- 
shire to  Yorkshire  and  the  more  northern  counties,  we  find  the 
Carboniferous  group  beginning  gradually  to  assume  a  new  cha- 
racter, there  being  first  a  slight  intermixture  of  the  Coal-measures 
and  Mountain  limestone  at  their  contact,  and  these  alternations 
taking  place  afterwards  on  a  still  greater  scale.  The  Coal,  in 
Yorkshire,  does  not  cease  when  we  reach  the  Millstone-grit, 
although  it  is  there  in  diminished  quantity  ;  and  beneath  that 
grit  is  a  complex  deposit,  1000  feet  thick,  of  limestones,  alter- 
nating with  coal-bearing  sandstones  and  shale,  below  which 
comes  the  great  mass  of  mountain  limestone.*  In  Scotland  we 
observe  a  still  wider  departure  from  the  type  of  the  south  of 
England,  the  mixture  of  marine  limestone  with  sandstone  and 
shale,  containing  coal,  being  more  complete. 

The  importance  of  the  coal  in  England,  considered  economi- 
cally, is  greatly  enhanced  by  the  rich  beds  of  iron-ore  which 
occur  in  the  associated  shales,  and  the  contiguity  as  a  flux  to  re- 
duce the  iron-ore  to  a  metallic  state,  f 

It  is  now  generally  admitted,  that  all  coal  is  of  vegetable  ori- 
gin, the  vegetable  structure  being  still  recognizable  in  many  kinds 
£>f  coal,  when  slices  thin  enough  to  transmit  light  are  obtained 
and  examined  by  the  miscroscope.  Impressions  also  of  plants, 
together  with  entire  trunks  of  trees,  are  frequently  met  with  in 
the  accompanying  shale  and  sandstone ;  leaves  also,  and  small 
branches,  and  fruits,  occur  in  nodules  of  clay-ironstone,  the  in- 
closed vegetable  having  served  as  a  nucleus  round  which  the 
ferruginous  matter,  usually  carbonate  of  iron,  has  concreted. 
Some  of  the  coal-measures  are  of  freshwater  origin,  and  many 
have  been  formed  in  lakes,  others  seem  to  have  been  deposited 
in  estuaries,  or  at  the  mouths  of  rivers,  in  spaces  alternately  oc- 
cupied by  fresh  and  salt  water. 

Thus  a  freshwater  deposit,  near  Shrewsbury,  has  been  ascer- 
tained by  Mr.  Murchison  to  be  the  youngest  member  of  the  car- 
boniferous series  of  that  district,  at  the  point  where  the  coal-mea- 
sures pass  into  the  lower  New  Red  formation.  It  consists  of 
shales  and  sandstones  about  150  feet  thick,  with  coal,  and  traces 


*  Sedgwick,   Geol.  Trans.,  Second  Series,  vol.  iv. ;  and   Phillips,    Geol.  of 
Yorksh.,  part  2. 
t  Conybeare,  Outlines,  &c.,  p.  333. 


PART  II.     CHAPTER  XX. 


245 


Fossils  of  the  Carboniferous  Group. 


Freshwater  fossils. — CoaZ. 


Fig.  244. 


245. 


a.  Microconchus 
carbonarius. 

b.  var.  of  same,  nat. 
size,  and  magnified. 


of  plants,  including  a  bed  of 
limestone,  varying  from  two 
to  nine  feet  in  thickness, 
which  is  cellular,  and  re- 
sembles the  lacustrine  lime- 
stone of  France  and  Ger- 
many. It  has  been  traced 
for  30  miles  in  a  straight 
line,  and  recognized  at  more 
distant  points.  The  charac- 
teristic fossils  are  a  small 
bivalve,  having  the  form  of 
cypris  inflate,  natural  a  cyclas,  a  small  cypris, 
size,  and  magnified.  (Fig.  245.)  and  a  miscrosco- 

Murchison.*  V      &  / 


pic  shell,  (microconchus)  of 
an  extinct  genus. 

But  in  the  lower  coal-measures  of  Coalbrook  Dale,  the  strata, 
according  to  Mr.  Prestwich,  often  change  completely  within  very 
short  distances,  beds  of  sandstone  passing  horizontally  into  clay, 
and  clay  into  sandstone.  The  coal-seams  often  wedge  out  or 
disappear;  and  sections,  at  places  nearly  contiguous,  present 
marked  lithological  distinctions.  In  this  single  field,  in  which 
the  strata  are  from  700  to  800  feet  thick,  between  40  and  50 
species  of  terrestrial  plants  have  been  discovered,  besides  several 
fishes  and  trilobites  ;  the  latter  distinct  in  form  from  those  occur- 
ring in  the  Silurian  strata.  Also  upwards  of  40  species  of  mol- 
lusca,  among  which  are  two  or  three  of  the.  fresh  water  genus 
Unio,  and  others  of  marine  forms  such  as  Nautilus,  Orthoceras, 
Spirifer,  and  Productus.  Mr.  Prestwich  suggests,  that  the  inter- 
mixture of  beds  containing  freshwater  shells  with  others  full  of 
marine  remains,  and  the  alternation  of  coarse  sandstone  and 
conglomerate  with  beds  of  fine  clay  or  shale  containing  the  re- 
mains of  plants,  may  be  explained  by  supposing  that  the  deposit 
of  Coalbrook  Dale,  originated  in  a  bay  of  the  sea  or  estuary  into 
which  flowed  a  considerable  river  subject  to  occasional  freshes.f 

In  the  Edinburgh  coal-field  at  Burdiehouse,  fossil  fishes,  mol- 
lusca  and  cypris,  very  similar  to  those  in  Shropshire  and  Staf- 
fordshire, have  been  found  by  Dr.  Hibbert.ij:  In  the  coal-field 
also  of  Yorkshire  there  are  freshwater  strata,  some  of  which 


*  Silurian  System,   p.  84. 

t  Prestwich,  Geol.  Soc.  Proceedings,  No.  46.  Murchison,  Silurian  System, 
p.  150. 

t  Trans.  Roy.  Soc.  Edin.  vol:  xiii.  Homer,  Edin.  New  Phil.  Journ.,  April 
1836. 


246 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Fossil  Fish  of  the  Coal. 


contain  shells  referred  to  the  genus  Unio ;  but  in  the  midst  of 
the  series  there  is  one  thin  but  very  widely  spread  stratum, 
abounding  in  marine  shells,  such  as  Ammonites  Listeri.  (Fig. 
246.)  Orthoceras,  Pecten  papyraceus,  (Fig.  247.)  and  seve- 
ral fishes.* 


Fig.  246. 


Fig.  247. 


Ammonites  Listeri,  Sow. 


Pecten  papyraceus,  Sow. 


Fig.  248. 


No  similarly  intercalated  layer  of  marine  shells  has  been 
noticed  in  the  neighbouring  coal-field  of  Newcastle,  where,  as  in 
South  Wales,  and  Somersetshire,  the  marine  deposits  are  entirely 
below  those  containing  terrestrial  and  freshwater  remains.f 
No  bones  of  mammalia  or  reptiles  have  as  yet  been  discovered 
in  strata  of  the  carboniferous  group.  The 
fish  are  numerous,  and  for  the  most  part 
very  remote  in  their  organization  from 
those  now  living,  as  they  belong  chiefly  to 
the  Sauroid  family  of  Agassiz ;  as  Mega- 
lichthys,  Holoptychus,  and  others,  which 
were  often  of  great  size,  and  all  predace- 
ous.  Their  osteology,  says  M.  Agassiz, 
reminds  us  in  many  respects  of  the  skele- 
tons of  saurian  reptiles,  both  by  the  close 
sutures  of  the  bones  of  the  skull,  their 
large  conical  teeth  striated  longitudinally, 
(see  Fig.  248.)  the  articulations  of  the  spi- 
nous  processes  with  the  vertebrae,  and 
other  characters.  Yet  they  do  not  form  a 
family  intermediate  between  fish  and  rep- 
tiles, but  are  true  fish.^. 

The  annexed  figure  represents  a  large 
tooth  of  the  Megalichthys,  found  by  Mr. 
Horner  in  the  Cannel  coal  of  Fifeshire. 
It  probably  inhabited  an  estuary,  frequenting  both  the  mouths  of 
rivers  and  the  sea. 


Megalichthys  Hibberti,  Ag. 

Edinburgh  coal-field ; 

natural  size. 


*  Phillips ;  art  "  Geology,"  Encyc.  Metrop.,  p.  590. 
t  Agassiz,  Poiss.  Foss.,  livr.  4.  p.  62.  and  livr.  5.  p.  88. 


t  Ibid.,  p.  592. 


PART  II.     CHAPTER  XX.  247 

Fossil  Plants  of  the  Coal  Strata. 

Fossil  Plants  of  the  Coal. — But  the  flora  of  the  coal  forms 
the  most  interesting  feature  in  its  palaeontology,  and  is  far  better 
known  to  us  than  any  other  flora  antecedent  to  the  tertiary  era. 
About  300  species  of  terrestrial  plants  are  enumerated  by  M. 
Adolphe  Brongniart  as  proper  to  the  Coal,  but  botanists  have 
encountered  the  greatest  difficulty  in  determining  the  natural 
affinities  of  these  fossils,  it  being  rare  to  find  in  them  any  vestige 
of  flower,  seed,  or  fruit,  those  organs  which  afford  the  most  con- 
venient characters  for  classifying  living  plants.  They  have  been 
obliged,  therefore,  first  to  study  more  minutely  the  different 
forms  of  bark  in  existing  trees,  their  various  modes  of  branch- 
ing, the  tissue  of  their  wood,  nervures  of  the  leaves,  and  other 
peculiarities  of  vegetable  structure  which  might  enable  them  to 
institute  a  direct  comparison  between  the  analogous  parts  of 
recent  and  fossil  plants.* 

The  most  common  of  these  vegetable  remains  may  be  provi- 
sionally classed  under  the  following  heads : — First,  Ferns  and 
Sigillarise ;  secondly,  Lepidodendra,  allied  to  Lycopodiacea  ? 
thirdly,  Calamites,  allied  to  Equisetacece  1  fourthly,  Coniferous 
plants  ;  fifthly,  Stigmarise,  apparently  an  extinct  family  of  plants. 

Ferns  and  Sigillaria. — The  leaves,  or  more  properly  speak- 
ing, the  fronds,  of  ferns,  (see  Figs.  249,  250.)  for  the  most  part 
destitute  of  fructification,  exceed  in  number  all  other  plants  in 
the  shale  of  the  coal.  They  have  been  divided  by  M.  Ad.  Brong- 
niart into  genera,  characterized  chiefly  by  the  branching  of  the 
fronds,  and  the  way  in  which  the  veins  of  the  leaves  are  dis- 
posed. These  fronds  are  often  accompanied  by  large  fluted 
stems  or  trunks  of  trees  which  have  been  squeezed  down  and 
flattened  as  they  lay  prostrate  in  the  shale,  so  that  the  opposite 
sides  meet,  but  which  when  they  occur  in  the  accompanying  grit 
or  sandstone,  and  are  placed  obliquely  or  vertically  to  the  planes 
of  stratification,  are  round  and  uncompressed.  Their  bark  has 
been  converted  into  coal ;  and  they  must  have  been  hollow  when 
first  deposited,  for  the  interior  became  filled,  not  only  with  sand, 
but  with  leaves  and  branches  of  ferns,  introduced  from  above. 
Impressions  of  these  fronds  are  now  frequent  in  the  pillars  of 
sandstone,  which  may  be  regarded  as  casts  of  the  interior  of 
those  ancient  trees.  Most  of  the  trunks  or  stems  now  alluded 
to  have  been  called  Sigillariee.  They  vary  from  half  a  foot  to 
five  feet  in  diameter,  and  must  have  been  sometimes  forty  or  fifty 
feet  high. 

It  is  admitted  by  all  botanists  that  some  of  these  gigantic 
stems,  all  of  which  are  comprehended  by  Brongniart  in  his 

*  See  the  works  of  MM.  Ad.  Brongniart,  Stern  berg,  and  others,  and  the  Fossil 
Flora  of  Lmdiey  and  Hut  ton. 


248 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Fossil  Plants  of  the  Coal  Strata. 


Fig.  249. 


Fiff.  250. 


Pecopteris  lonchittca. 
(Foss.  Flo.  153.) 


a.  Sphenopteris  crenata. 

b.  The  same,  magnified. 

(Foss.  Flo.  101.) 


Fig.  251. 


Fig.  252. 


Sigillaria  lAndleyi,  Brong. 
(Cavlopteris  primaxa,  Lindley.) 


Sigillaria  lavigata,  Brong. 


PART  II.     CHAPTER  XX.  249 

Recent  Tree-Ferns. 

genus  Sigillaria,  were  true  arborescent  ferns,  as  for  example, 
that  section  which  has  been  named  Caulopteris  by  Lindley  and 
Hutton.  (See  Fig.  251.)  But  these  are  comparatively  rare, 
whereas  of  the  other  section  (Fig.  252.)  more  than  forty  species 
have  been  described.  In  these  the  scars  on  the  stem  are  smaller 
and  more  regularly  arranged  in  parallel  series  on  the  fluted 
bark.  (Fig.  252.) 

The  recent  tree-ferns  belong  to  one  tribe  (Polypodiacece.}  and 
to  a  small  number  only  of  genera  in  that  tribe,  in  all  of  which 
the  surface  of  the  trunk  is  marked  with  scars,  or  cicatrices,  left 
after  the  fall  of  the  fronds.  These  scars  are  precisely  similar  to 
those  of  Caulopteris  (Fig.  251);  but  Mr.  Lindley  objects  to  the 
opinion  that  the  remaining  Sigillarise  of  Brongniart  were  Tree- 


Fig.  253.  Fig.  254.  Fig.  255. 

Living  Tree-ferns  of  different  genera.  (Ad.  Brong.) 

Fig.  253.  Tree-fern  from  Isle  of  Bourbon. 
Fig.  254.  Cylhea  glauca,  Mauritius. 
Fig.  255.  Tree-fern  from  Brazil. 

ferns,  because  the  scars  in  these  are  smaller,  dissimilar  in  form,  and 
more  regularly  arranged  in  parallel  lines ;  also,  because  the  stems 
are  fluted,  (see  Fig.  252.)  and  sometimes  bifurcating.  M.  Brong- 
niart has  replied,  that  the  forking  of  the  stems  of  some  of  the 
fossil  trees  is  no  more  than  might  have  been  expected  from  their 
large  size ;  and  as  to  the  forms  of  the  discs  or  scars  from  which 
the  fronds  have  fallen,  their  individual  variations  are  not  greater 


250  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Fossil  Plants  of  the  Coal  Strata. 

than  those  which  we  find  in  the  fronds  of  different  genera  of 
living  ferns,  which  do  not  in  the  present  state  of  the  globe  attain 
the  size  of  trees. 

Lepidodendra.  —  Another  class  of  fossils,  very  common  in 
the  coal -shales,  have  been  named  Lepidodendra.  Some  of  these 
are  of  small  size,  and  approach  very  near  in  form  to  the  modern 


Fig.  256.  Fig.  257.  Fig.  258. 

Lepidodendron  Sternbergii.    Coal-measures,  near  Newcastle. 

Fig.  256.  Branching  trunk,  49  feet  long,  supposed  to  have  belonged  to  L.  Stern- 
bergii.  (Foss.  Flo.  203.) 

Fig.  257.  Branching  stem  with  bark  and  leaves  of  JL.  Sternbergii.  (Foss.  Flo.  4.) 
Fig.  258.  Portion  of  same  nearer  the  root ;  natural  size.  (Ibid.) 

Lycopodiums,  or  club-mosses,  while  others  of  much  larger 
dimensions  are  supposed  to  have  been  intermediate  between  these 
and  coniferous  plants.  The  annexed  figures  represent  a  large 
fossil,  Lepidodendron,  forty-nine  feet  long,  lately  found  in  Jar- 
row  Colliery,  near  Newcastle,  lying  in  shale  parallel  to  the 
planes  of  stratification.  Fragments  of  others,  found  in  the  same 
shale,  indicate  by  the  size  of  the  rhomboidal  scars  which  cover 
them  a  still  greater  magnitude.  The  living  club-mosses,  of 
which  there  are  about  200  species,  are  abundant  in  tropical  cli- 
mates, where  one  species  is  sometimes  met  with  attaining  a 
height  of  three  feet.  They  usually  creep  on  the  ground,  but 
some  stand  erect,  as  the  L.  densum,  from  New  Zealand.  (Fig. 
259.) 

Calamites. — These  fossils  have  a  jointed  stem,  longitudinally 
striated,  and  are  supposed  by  M.  Brongniart  to  have  been  allied 
to  the  Equisetacea,  or  horse-tail  tribe ;  aquatic  plants  which,  in 
a  living  state,  are  only  two  or  three  feet  high  in  our  climates, 


PART  II.     CHAPTER  XX. 


251 


Fossil  Plants  of  the  Coal  Strata. 


a.  Lycopodium  densum;  banks  of  R.  Thames,  New  Zealand. 
I.  branch,  natural  size.  c.  part  of  same,  magnified. 

and  even  in  tropical  countries  only  attain,  as  in  the  case  of 
Equisetum  giganteum,  discovered  by  Humboldt  and  Bonpland, 
in  South  America,  a  height  of  about  five  feet,  the  stem  being  an 
inch  in  diameter.  The  Calamites,  however,  of  the  Coal  differed 
from  these,  principally  in  being  furnished  with  a  thin  bark, 
which  is  represented  in  the  stem  of  C.  Suckowii,  (Fig.  261.)  in 
which  it  will  be  seen  that  the  striped  external  pattern  does  not 


Fig.  260. 


Fig.  261. 

HKillllllil 


Calamites  canaformis,  Schlot. 
(Foss.  Flo.  79.)  Common  in 
English  coal. 


Calamites  Suckowii,  Brong. 
natural  size.  Common  in 
coal  throughout  Europe. 


agree  with  that  left  on  the  stone  where  the  bark  is  stripped  off, 
so  that  if  the  two  impressions  were  seen  separately,  they  might 
be  mistaken  for  two  distinct  species. 

Coniferce. — The  structure  of  the  wood  of  certain  coal-plants 
displays  so  great  an  analogy  to  that  of  certain  pines  of  the  genus 
Araucaria,  as  to  lead  to  the  opinion  that  some  species  of  fira 
existed  at  this  period.  (See  above,  p.  56.) 


252 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Fossil  Plants  of  the  Coal  Strata. 


Fig.  262. 


Fig.  263. 


Stigmaria  ficoides,  Brong.     One-fourth  of  nat.  size.     (Foss.  Flo.  32.) 

StigmaricB. — Fragments  of  a  plant  which  has  been  called 
Stigmaria  Jicoides^  occur  in  great  numbers  in  almost  every 
coal-pit.  It  is  supposed  to  have  been  a  huge  succulent  water- 
plant  of  an  extinct  family ;  thin  transparent  sections  of  the  stem 
exhibiting  an  anatomical  structure  quite  different  from  the  wood 
of  any  living  tree.*  According  to  the  conjectures  of  some  bota- 
anists,  it  approached  most  nearly  to  the 
family  Lycopodiacecz ;  according  to 
others,  to  Euphorbiacete.  Mr.  Hutton 
discovered  one  of  these  Stigmarise  form- 
ing a  huge  dome-shaped  body,  from  which 
.  ._._  .  .  twelve  branches  spread  horizontally  in 

Surface  of  another  individual  of      „   ,.  n       v    •  i-         • 

same  species,  showing  form  of  all  directions,  each,  usually  dividing  into 
tubercles.   (Foss.  Fio.  34.)      two  armSj  from  twenty  to   thirty   feet 
long,  to  which  leaves  of  great  length  were  attached.     Dr.  Buck- 
land  imagines  these  plants  to  have  grown  in  swamps,  or  to  have 
floated  in  lakes  like  the  modern  Stratiotes.f 

I  shall  postpone  some  general  remarks  on  the  climate  of  the 
Carboniferous  period,  arising  out  of  the  contemplation  of  its  flora, 
until  something  has  been  said  of  the  contemporaneous  Mountain 
limestone  and  its  marine  fossils. 


Lindley,  Foss.  Flora,  p.  166. 


t  Bridgew.  Treat,  p.  478. 


PART  II.     CHAPTER  XXI.  253 


Mountain  Limestone. 


CHAPTER  XXI. 

CARBONIFEROUS  GROUP  Continued,  AND  OLD  RED  SANDSTONE. 

Corals  and  shells  of  the  Mountain  limestone — Hot  climate  of  the  Carboniferous 
period  inferred  from  the  marine  fossils  of  the  Mountain  limestone  and  the  plants 
of  the  Coal — Origin  of  the  Coal-strata — Contemporaneous  freshwater  and  marine 
deposits — Modern  analogy  of  strata  now  in  progress  in  and  around  New  Zealand 
—  Vertical  and  oblique  position  of  fossil  trees  in  the  Coal  —  How  enveloped  — 
How  far  they  prove  a  rapid  rate  of  deposition — Old  Red  sandstone — its  subdi- 
visions— its  fossil  shells  and  fish. 

CARBONIFEROUS  or  Mountain  limestone. — We  have  already 
seen  that  this  rock  lies  sometimes  entirely  beneath  the  Coal- 
measures,  while  in  other  districts,  it  alternates  with  the  shales 
and  sandstone  of  the  Coal.  In  both  cases  it  is  destitute  of  land 
plants,  and  usually  charged  with  corals,  which  are  often  of  large 
size ;  and  several  species  belong  to  the  lamelliferous  class  of  La- 
marck, which  enter  largely  into  the  structure  of  coral  reefs  now 
growing.  There  are  also  a  great  number  of  Crinoidea,  and  a  few 
Echinida,  associated  with  the  zoophytes  above  mentioned.  The 
Brachiopoda  constitute  a  large  proportion  of  the  Mollusca,  many 
species  being  referable  to  two  extinct  genera,  Spirifer  (or  Spiri- 
fera)  (Fig.  264.)  and  Producta  (Fig.  265.).  There  are  also 

Fig.  264.  Fig.  265. 


Spirifera  glabra,  Sow*  Producta  Martini,SowJ 

Mountain  limestone.  Mountain  limestone. 

many  univalve  and  bivalve  shells  of  existing  genera  in  the 
Mountain  limestone,  such  as  Turritella,  Buccinum,  Patella,  Iso- 
cardia,  Nucula,  and  Pecten.ij:  But  the  Cephalopoda  depart,  in 

*  Phillips,  Geol.  of  Yorksh.  pi.  10.  fig.  11. 

t  Ibid.,  pi.  8.  fig.  19.  f  Ibid.,  vol.  ii.  p.  208. 


254 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Corals  and  Shells  of  the  Mountain  Limestone. 


general,  more  widely  from  living  forms,  some  being  generically 
distinct  from  all  those  found  in  strata  newer  than  the  Coal.  In 
this  number  may  be  mentioned  Orthoceras,  a  siphuncled  and 
chambered  shell,  like  a  Nautilus  uncoiled  and  straightened. 
Some  species  of  this  genus  are  several  feet  long  (Figs.  266, 


.  266. 


Fig.  257. 


Portion  of  Orthoceras  laterale,  Phillips. 
Mountain  Limestone. 


O.  giganteum,  Sow. 

Section  showing  the  siphuncJe 

reduced  two-thirds. 


267.).  The  Goniatite  is  another  genus  nearly  allied  to  the  Am- 
monite, from  which  it  differs  in  having  the  lobes  of  the  septa  free 
from  lateral  denticulations,  or  crenatures ;  so  that  the  outline  of 
these  is-  continuous  and  uninterrupted  (see  a,  Fig.  268.).  Their 


Fig.  268. 


Fig.  269. 


Goniatites  evolutus,  Phillips.* 
Mountain  limestone. 


Bellerophon  costatus,  Sow.f 
Mountain  limestone. 


siphon  is  small,  and  in  the  form  of  the  strise  of  growth  they 
resemble  Nautili.  Another  extinct  generic  form  of  Cephalopod, 
abounding  in  the  Mountain  limestone,  and  not  found  in  strata  of 
later  date,  is  the  Bellerophon  (Fig.  269.),  of  which  the  shell, 
like  the  living  Argonaut,  was  without  chambers. 

Climate  of  the  Carboniferous  period. — The  abundance  of 
lamelliferous  and  other  corals,  of  large  chambered  Cephalopods 
and  Crinoidea,  naturally  leads  us  to  infer  that  the  waters  of  the 
sea,  at  this  period,  were  of  a  far  warmer  and  more  equable  tem- 
perature than  is  now  experienced  in  those  latitudes  where  the 
Coal  strata  abound,  in  Europe.  M.  Adolphe  Brongniart  has  teen 
led  to  a  similar  conclusion  in  regard  to  the  temperature  of  the 


*  Phillips,  Geol.  of  Yorksh.,  pi.  20.  fig.  65. 


tlbid.pl.  17.  fig.  15. 


PART  II.    CHAPTER  XXI.  255 

Climate  of  the  Carboniferous  Period. 

air  from  considering  the  Carboniferous  flora.  The  unquestioned 
existence  of  large  tree-ferns,  such  as  Caulopteris  (Fig.  251.)  now 
exclusively  the  inhabitants  of  hot  and  humid  climates,  and  the 
great  variety  of  fossil  fronds  of  ferns  in  the  Coal  confirm  this 
idea,  even  if  we  refuse  to  accede  to  the  arguments  adduced  to 
prove  that  SigillariaB  were  tree-ferns  of  extinct  genera.  The 
same  views  receive  farther  countenance,  if  the  Lepidodendra  and 
Calamites  are  rightly  conjectured  to  have  been  gigantic  plants  of 
the  orders  LycopodiacecB  and  Equisetacece,  which,  although 
most  largely  developed  at  present  in  the  tropical  zone,  are  even 
there  of  pigmy  stature  in  comparison  with  the  fossil  tribes  just 
alluded  to.  The  Araucaria,  also,  is  a  family  of  pines  now  met 
with  in  temperate  and  warm  latitudes ;  and  the  fir  trees  proper  to 
the  forests  of  arctic  regions  do  not  appear  to  have  any  fossil  re- 
presentatives in  the  Coal.  M.  Ad.  Brongniart,  when  endeavouring 
to  establish  the  great  heat  and  moisture  of  the  climate  of  the  era 
under  consideration,  may  perhaps  have  relied  too  much  on  the 
numerical  preponderance  of  ferns  over  other  orders  of  coal- 
plants.  We  may  easily  be  deceived  by  such  reasoning,  because 
it  is  founded  on  negative  facts,  or  the  absence  of  plants  of  certain 
orders,  families,  and  genera.  On  this  subject  Professor  Lindley 
has  observed,  that  the  small  variety  in  the  forms  of  each  fossil 
flora  must,  in  a  great  degree,  depend  on  the  relative,  destructi- 
bility  of  plants  when  suspended  in  water  before  they  are  imbedded 
in  strata.  In  illustration  of  this  point,  he  threw  into  a  vessel 
containing  fresh  water  177  plants,  among  which  were  species 
of  all  the  orders  found  in  the  Carboniferous  flora,  with  others 
representing  the  remaining  families  and  natural  orders  in  the 
living  creation,  and  found  that,  at  the  end  of  two  years,  all  had 
decayed  and  disappeared  except  the  ferns,  palms,  Lycopodia- 
CCCB  and  Conifertz.  The  fructification  of  the  ferns  had  also  van- 
ished, but  the  form  and  nervures  of  the  leaves  remained.* 

No  inference,  however,  drawn  from  this  experiment,  can  en- 
tirety explain  away  the  fact  of  the  vast  preponderance  in  the 
coal-shales  of  fern-leaves  over  those  of  Dicotyledonous  plants. 
Impressions  of  these  last,  together  with  their  wood,  are  plentifully 
preserved  in  tertiary  rocks  in  which  fossil  ferns  are  rare ;  and 
had  they  been  drifted  down  in  as  large  numbers  as  ferns  into  the 
estuaries  of  the  Carboniferous  period,  they  would  have  left  im- 
pressions of  their  shape  in  shale  and  sandstone,  as  they  have 
done  in  more  recent  formations. 

It  would,  moreover,  be  rash  to  assume  that  the  coal-plants  in 
general  floated  about  in  water  for  a  year  or  two  before  they  were 

*  Lindley,  FOES.  Flora,  part  17. 


256      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Origin  of  the  Coal  Strata. 

enveloped  in  sediment.  It  is  more  probable  that  a  large  part  of 
them  were  deposited  immediately  with  the  mud  and  sand  swept 
down  with  them  by  rivers  into  lakes  or  the  sea.  This  must  have 
happened  in  those  rare  cases  where  the  ferns  still  retain  their 
fructification.  Where  this  has  disappeared,  its  decomposition 
may  often  have  been  subsequent  to  the  inclosure  of  the  frond  in 
mud  or  sand. 

Origin  of  the  Coal  strata. — Detached  portions  of  the  ancient 
Carboniferous  group  extend  from  Central  Europe  to  Melville 
Island  and  the  confines  of  the  arctic  region ;  but  do  not  appear 
in  the  south  of  Europe ;  for  the  lignite  and  coal  found  south  of 
the  Alps  and  Pyrenees,  in  Spain, Italy,  Greece,  and  other  coun- 
tries bordering  the  Mediterranean,  seem  referable  to  the  Creta- 
ceous and  other  comparatively  modern  groups. 

It  has  been  already  shown  that,  in  some  parts  of  England,  as 
in  Shropshire,  certain  Coal-measures  consist  of  freshwater  strata, 
and  may  have  originated  in  a  lake,  while  others,  not  far  distant, 
were  deposited  in  estuaries  to  which  the  sea  obtained  access 
occasionally ;  while  a  third  class  were  formed  at  the  bottom  of 
an  open  sea,  or  in  bays  of  salt  water  into  which  land  plants  were 
drifted.* 

In  many  parts  of  France  and  Germany  there  are  isolated 
patches  of  Coal  strata,  entirely  free  from  marine  fossils,  which 
repose  on  granite  and  other  hypogene  rocks.  They  are  often 
confined  to  an  extremely  small  area,  as  at  St.  Etienne,  in  the 
department  of  the  Loire ;  at  Brassac,  in  that  of  Puy  de  Dome ; 
at  Sarrebruck ;  also  in  Silesia ;  and  a  hundred  other  places. 
All  these  deposits  may  have  been  formed  in  lakes,  existing  in 
the  islands  of  that  sea  in  which  the  Mountain  limestone  was 
formed,  f 

If  the  climate  of  New  Zealand  and  the  surrounding  ocean  was 
warmer,  so  that  tree-ferns  could  thrive  more  luxuriantly  on  the 
land,  and  corals  build  reefs  in  the  sea,  we  might  conceive  new 
strata  to  accumulate  in  that  part  of  the  globe  analogous  to  those 
of  the  ancient  Coal.  The  two  islands  of  New  Zealand  are  be- 
tween 800  and  900  miles  in  length ;  and  through  the  middle  of 
them  runs  a  lofty  chain  of  mountains,  said  in  some  parts  to  be 
14,000  feet  high,  and  covered  with  perpetual  snow.  Many 
rivers  descend  from  their  sides ;  and,  in  the  spring,  these  are 
copiously  charged  with  sediment,  and  with  abundance  of  drift 
wood.  Opposite  the  mouths  of  these  rivers,  and  near  the  shores, 
wherever  these  may  be  wasting  by  the  action  of  the  waves,  an 
irregular  zone  of  gravel,  sand,  and  mud,  must  be  forming  in  the 

*  Murchison,  Silurian  System,  p.  148.      t  Burat's  D'Aubuisson,  torn.  ii.  p.  26a 


PART  II.     CHAPTER  XXL  257 

Erect  Position  of  Trees  in  the  Coal  Strata. 

surrounding  sea — a  zone  several  thousand  miles  in  circumference. 
No  less  than  57  species  of  ferns,  some  few  of  them  arborescent, 
have  been  already  discovered  in  this  country ;  and  what  is  re- 
markable, one  tree-fern  ranges  in  this  country  as  far  south  as  the 
46th  degree,  south  latitude.  There  are  no  indigenous  mammalia 
except  one  rat,  and  a  species  of  bat ;  few  reptiles,  and  none  of 
large  size;  so  that  we  may  anticipate  a  total  absence  of  the 
bones  of  land  quadrupeds,  and  a  scarcity  of  those  of  reptiles,  in 
the  modern  estuary  and  lacustrine  deposits  of  this  region.  That 
there  are  lacustrine  strata  now  in  progress  is  certain,  since  one 
lake  called  Rotorua,  in  the  interior  of  the  northern  island,  is  said 
to  be  40  miles  long,  and  receives  the  waters  of  many  small  rivers 
and  torrents.* 

The  minor  repetitions  of  alternate  fresh  and  saltwater  strata 
in  the  Coal,  have  been  ascribed  to  such  changes  as  may  annu- 
ally occur  near  the  mouths  of  rivers ;  but  when  shale  and  grit, 
containing  coal  and  freshwater  shells,  are  covered  by  large  masses 
of  coralline  rock,  and  these  again  by  other  Coal-measures,  we 
must  suppose  great  movements  of  elevation  and  subsidence,  like 
those  by  which  I  endeavoured  to  explain,  in  Chapters  XVI.  and 
XVIIL,  the  superposition  of  the  Cretaceous  group  to  the  Wealden, 
or  the  alternations  of  argillaceous  and  calcareous  rocks  in  the 
Oolite.  In  adopting  such  views,  we  must  suppose  the  lapse  of 
vast  periods  of  time ;  as  the  thickness  of  the  Coal  strata,  in  some 
parts  of  England,  independently  of  the  Mountain  limestone,  has 
been  estimated  at  3000  feet.  Besides,  we  can  by  no  means  pre- 
sume that  all  coal-fields  were  in  progress  at  once,  much  less  that, 
in  the  same  field,  each  mass  of  strata  which  is  parallel,  or  occu- 
pies a  corresponding  level,  was  formed  simultaneously.  It  is 
far  more  consistent  with  analogy  to  suppose  that  rivers  filled  up 
first  one  part  of  a  fiord,  gulf,  or  bay,  nearest  the  land,  and  then 
another ;  so  that  the  sea  was  gradually  excluded  from  certain 
spaces  which  it  previously  occupied.  This  is  doubtless  the  cause 
why  the  coal-bearing  strata  are  generally  uppermost,  and  the 
Mountain  limestone  the  lowest  part  of  each  series ;  and  why,  in 
certain  districts  in  the  S.  W.  of  England,  the  Mountain  limestone 
suddenly  thins  out,  so  that  coal-shales  and  grit  rest  immediately 
upon  older  and  unconformable  rocks. 

Erect  position  of  fossil  trees  in  the  Coal  strata. — A  great 
number  of  the  fossil  trees  of  the  Coal  are  in  a  position  either  ob- 
lique or  perpendicular  to  the  planes  of  stratification.  This  singu- 
lar fact  is  observed  on  the  Continent  as  well  as  in  England,  and 
merits  great  attention,  not  only  as  opening  a  curious  field  for 

*  Account  of  New  Zealand,  published  for  New  Zealand  Association. 

W* 


258 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Erect  Position  of  Trees  in  the  Coal  Strata. 


speculation,  but  because  it  has  furnished  a  popular  argument  to 
some  writers  who  desire  to  prove  the  earth's  crust  to  be  no  more 
than  5000  or  6000  years  old.  The  fact  did  not  escape  the  notice 
of  Werner,  who  conceived  that  the  trees  must  have  lived  on  the 
spots  where  they  are  now  found  fossil ;  and  this  hypothesis  was 
defended  by  M.  Alexandre  Brongniart,  in  the  account  given  by 
him,  in  1821,  of  the  coal-mine  of  Treuil,  at  St.  Etienne.  near 
Lyons.*  (Fig.  270.)  In  this  mine,  horizontal  Coal  strata  aro 

Fig.  270. 


Section  showing  the  erect  position  of  fossil  trees  in  coal-sandstone  at 
St.  Etienne.    (Alex.  Brongniart.) 

traversed  by  vertical  trunks  of  Monocotyledonous  vegetables  re- 
sembling bamboos,  or  large  Equiseta.  These  beds  are  represent- 
ed in  the  above  figure  (270.),  and  are  from  10  to  13  feet  in 
height,  consisting  of  micaceous  sandstone,  distinctly  stratified, 
and  passing  into  the  slaty  structure.  Since  the  consolidation  of 
the  stone,  there  has  been  here  and  there  a  sliding  movement, 
which  has  broken  the  continuity  of  the  stems,  throwing  the  upper 
parts  of  them  on  one  side,  so  that  they  are  often  not  continuous 
with  the  lower. 

Now,  had  these  trees,  as  some  geologists  contend,  once  formed 
part  of  a  submerged  forest  like  that  of  Portland,  before  described, 


Annales  des  Mines,  1821. 


PART  II.    CHAPTER  XXL  259 

Erect  Position  of  Trees  in  the  Coal  Strata. 

(see  p.  205.)  all  the  roots  would  have  been  in  the  same  stratum, 
or  would  have  been  confined  to  certain  levels,  and  not  scattered 
irregularly  through  the  mass.  Besides,  when  the  stems  have 
any  roots  attached  to  them,  which  happens  but  rarely,  they  are 
imbedded  in  sandstone  precisely  similar  to  that  in  which  the 
trunks  are  inclosed,  there  being  no  soil  of  different  composition 
like  the  Portland  dirt-bed, — no  line  of  demarcation,  however 
slight,  between  the  supposed  ancient  surface  of  dry  land  and  the 
sediment  now  enveloping  the  trees. 

Some  may,  perhaps,  think  it  superfluous  to  advance  such 
objections  to  M.  Brongniart's  theory,  since  Dr.  Buckland  has 
informed  us  that,  when  he  visited  these  same  quarries  of  Treuil 
in  1826,  he  saw  so  many  trunks  in  an  inclined  posture,  that  the 
occasional  verticality  of  others  might  be  accidental.*  Neverthe- 
less, the  possibility  of  so  many  of  them  having  remained  in  an 
upright  posture  demands  explanation ;  and  there  are  analogous 
cases  on  record  respecting  similar  fossils  in  Great  Britain  of  a 
still  more  extraordinary  nature. 

In  a  colliery  near  Newcastle,  say  the  authors  of  the  Fossil 
Flora,  a  great  number  of  Sigillarias  were  placed  in  the  rock  as 
if  they  had  retained  the  position  in  which  they  grew.  Not  less 
than  30,  some  of  them  4  or  5  feet  in  diameter,  were  visible, 
within  an  area  of  50  yards  square,  the  interior  being  sandstone, 
and  the  bark  having  been  converted  into  coal.  The  roots  of  one 
individual  were  found  imbedded  in  shale ;  and  the  trunk,  after 
maintaining  a  perpendicular  course  and  circular  form,  for  the 
height  of  about  10  feet,  was  then  bent  over  so  as  to  become 
horizontal.  Here  it  was  distended  laterally,  and  flattened  so  as 
to  be  only  one  inch  thick,  the  flutings  being  comparatively  dis- 
tinct.f  Such  vertical  stems  are  familiar  to  our  miners,  under 
the  name  of  coal-pipes.  One  of  them,  72  feet  in  length,  was 
discovered,  in  1829,  near  Gosforth,  about  five  miles  from  New- 
castle, in  coal-grit,  the  strata  of  which  it  penetrated.  The  exte- 
rior of  the  trunk  was  marked  at  intervals  with  knots,  indicating 
the  points  at  which  branches  had  shot  off.  The  wood  of  the 
interior  had  been  converted  into  carbonate  of  lime;  and  its 
structure  was  beautifully  shown  by  cutting  transverse  slices,  so 
thin  as  to  be  transparent.  (See  p.  56.) 

In  1830,  a  slanting  trunk  was  exposed  in  Craigleith  quarry, 
near  Edinburgh,  the  total  length  of  which  exceeded  60  feet.  Its 
diameter  at  the  top  was  about  7  inches,  and  near  the  base  it 
measured  5  feet  in  its  greater,  and  2  feet  in  its  lesser  width.  The 
bark  was  converted  into  a  thin  coating  of  the  purest  and  finest 

*  Bridgew.  Treat.,  p.  471.         t  Lindley  and  Hutton,  Foss.  Flo.,  part  6.  p.  150. 


260 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


&ate  of  Deposition  of  the  Coal  Strata. 


Fig.  271. 


Inclined  position  of  a  fossil  tree  cutting  through 
horizontal  beds  of  sandstone,  Craigleith  quarry. 
Edinburgh.  Angle  of  inclination  from  a  to 
b,  27°. 


coal,  forming  a  striking 
contrast  in  colour  with  the 
white  quartzose  sandstone 
in  which  it  lay.  The  an- 
nexed figure  represents  a 
portion  of  this  tree,  about 
15  feet  long,  which  I  saw 
exposed  in  1830,  when  all 
the  strata  had  been  re- 
moved from  one  side.  The 
beds  which  remained  were 
so  unaltered  and  undis- 
turbed at  the  point  of 


junction,  as  clearly  to  show  that  they  had  been  tranquilly  depo- 
sited round  the  tree,  and  that  the  tree  had  not  subsequently 
pierced  through  them,  while  they  were  yet  in  a  soft  state.  They 
were  composed  chiefly  of  siliceous  sandstone,  for  the  most  part 
white ;  and  divided  into  laminae  so  thin,  that  from  six  to  four- 
teen of  them  might  be  reckoned  in  the  thickness  of  an  inch. 
Some  of  these  thin  layers  were  dark,  and  contained  coaly  mat- 
ter; but  the  lowest  of  the  intersected  beds  were  calcareous. 
The  tree  could  not  have  been  hollow  when  imbedded,  for  the 
interior  still  preserved  the  woody  texture  in  a  perfect  state,  the 
petrifying  matter  being,  for  the  most  part,  calcareous.*  It  is 
also  clear,  that  the  lapidifying  matter  was  not  introduced  late- 
rally from  the  strata  through  which  the  fossil  passes,  as  most  of 
these  were  not  calcareous.  It  is  well  known  that,  in  the  Missis- 
sippi and  other  great  American  rivers,  where  thousands  of  trees 
float  annually  down  the  stream,  some  sink  with  their  roots  down- 
wards, and  become  fixed  in  the  mud.  Thus  placed,  they  have 
been  compared  to  a  lance  in  rest ;  and  so  often  do  they  pierce 
through  the  bows  of  vessels  which  run  against  them,  that  they 
render  the  navigation  extremely  dangerous.  But  the  vertical 
coal-plants  did  not  always  retain  their  roots.  Perhaps  they  sank 
with  their  larger  end  downwards,  because  the  specific  gravity  of 
the  wood  may  have  been  greatest  near  the  lower  end.  In  trees 
of  the  Endogenous  class,  in  particular,  the  wood  of  the  inferior 
and  older  part  of  the  trunk  is  more  dense  than  the  upper  and 
younger  portions ;  and  if  the  former  should  become  water-logged 
while  the  upper  part  of  the  stem  still  remained  nearly  as  light 
as  water,  or  even  lighter,  not  only  would  the  whole  trunk  descend 
perpendicularly,  but  when  it  reached  the  bottom  it  might  stand 
upright,  provided  a  very  slight  support  was  afforded  to  its  lower 


See  figures  of  texture,  Witham,  Foss.  Veget.,  pi.  3. 


PART  II.     CHAPTER  XXI. 


Rate  of  Deposition  of  the  Coal  Strata. 


extremity  by  penetrating  to  the  depth  of  a  foot  or  two  into  soft 
mud.  How  long  such  trunks,  if  constantly  submerged,  might 
resist  decomposition,  is  a  question  which  cannot,  perhaps,  be 
determined  ;  but,  judging  from  the  duration  of  wooden  piles 
constantly  covered  by  water,  and  trees  naturally  submerged,  like 
those  in  Louisiana,*  we  may  conclude  that  they  might  endure 
for  many  years,  so  that  their  envelopment  in  strata,  like  those 
of  the  Coal,  may  have  been  effected  without  a  very  rapid  rate 
of  deposition. 

If,  however,  we  assume  that  strata  30  or  40  feet  thick  were 
often  thrown  down  in  a  few  years,  months,  or  even  days,  this 
fact  affords  no  ground  for  calculating  the  time  required  for  the 
formation  of  a  wide  coal-field. 

Suppose,  for  example,  the  structure  of  a  coal-field  always  re- 
sembled that  exhibited  in  the  annexed  section  (Fig.  272.),  we 
might  then  infer,  that  if  the  lowest  set  of  strata,  a,  having  a 
thickness  of  fifty  feet,  required  half  a  century  for  its  accumula- 
tion, the  strata,  a,  &,  c,  constituting  the  entire  coal-field,  and  being 
150  feet  thick,  might  have  been  completed  in  a  century  and  a 

Fig.  272. 


half.  But  as  the  beds  are  wedge-shaped,  and  often  thin  out ;  and 
as  the  successive  beds  of  a  single  coal-field  are  usually  arranged 
in  the  form  of  a,  6,  c,  d,  e  (Fig.  273.),  we  cannot  calculate  their 

Fig.  273. 


number  from  considering  any  one  section.  The  deposits,  a,  &, 
c,  tZ,  e,  traced  in  a  given  direction,  may  have  taken  each  fifty 
years  for  their  deposition  ;  but  they  may  have  been  as  limited  in 
breadth  as  in  length.  They  may  have  constituted  originally  a 
narrow  strip  of  land  like  part  of  the  delta  formed  by  the  Missis- 
sippi, since  New  Orleans  was  built,  by  the  incessant  discharge 
of  mud  and  drift  timber  into  the  Gulf  of  Mexico.  Although  by 
this  means  a  narrow  tongue  of  land  has  been  made  to  protrude 
for  several  leagues  into  the  sea,  yet  thousands  of  years  may 

*  See  Principles  of  Geology,  Index,  "  Bistineau." 


262      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Old  Red  Sandstone. 

elapse  before  a  square  area  of  low  land,  having  a  diameter  of  as 
many  leagues,  can  be  gained  from  the  Gulf  of  Mexico. 

OLD  RED  SANDSTONE. 

It  was  stated  that  the  Carboniferous  formation  was  surmount- 
ed by  one  called  the  "  New  Red  Sandstone,"  and  underlaid  by 
another  called  the  Old  Red,  which  last  was  formerly  merged  in 
the  Carboniferous  system,  but  is  now  found  to  be  distinguishable 
by  its  fossils.  The  Old  Red  Sandstone  is  of  enormous  thickness 
in  Herefordshire,  Worcestershire,  Shropshire,  and  South  Wales, 
where  it  is  seen  to  crop  out  from  beneath  the  Coal-measures  and 
to  repose  upon  the  Silurian  rocks.  In  that  region  its  thickness 
has  been  estimated  by  Mr.  Murchison  at  no  less  than  10,000 
feet.  It  consists  there  of 

1st  A  quartzose  conglomerate  passing  downwards  into  chocolate-red  and 
green  sandstone  and  marl. 

2d.  Cornstone  and  marl  (red  and  green  argillaceous  spotted  marls,  with  irre- 
gular courses  of  impure  concretionary  limestone,  provincially  called 
Cornstone,  mottled,  red,  and  green ;  remains  of  fishes). 

3d.  Tilestone  (finely  laminated  hard  reddish  or  green  micaceous  or  quartzose 
sandstones,  which  split  into  tiles ;  remains  of  mollusca  and  fishes). 

I  have  already  observed  that  the  fossils  are  rare  in  marls  and 
sandstones,  in  which  the  red  oxide  of  iron  prevails  ;  in  the  Corn- 
stone,  however,  of  the  counties  above-mentioned,  fishes  of  the 
genera  Cephalaspis  and  Onchus  have  been  discovered.*  In  the 
Tilestones  also,  Ictrfyodorulites,  of  the  genus  Onchus,  have  been 
obtained ;  and  a  species  of  Dipterus,  with  mollusca  of  the  genera 
Avicula,  Area,  Cucullsea,  Terebratula,  Lingula,  Turbo,  Trochus, 
Turritella,  Bellerophon,  Orthoceras,  and  others.f 

By  consulting  geological  maps,  the  reader  will  perceive  that 
from  Wales  to  the  north  of  Scotland,  the  Old  Red  sandstone 
appears  in  patches,  and  often  in  large  tracts.  Many  fishes  have 
been  found  in  it  at  Caithness,^:  and  various  organic  remains  in 
the  northern  part  of  Fifeshire,  where  it  crops  out  from  beneath 
the  Coal  formation,  and  spreads  into  the  adjoining  southern  half 
of  Forfarshire ;  forming,  together  with  trap,  the  Sidlaw  hills 
and  valley  of  Strathmore.  (See  section,  p.  99.)  A  large  belt 
of  this  formation  skirts  the  southern  borders  of  the  Grampians, 
from  the  sea-coast  at  Stonehaven  and  the  Frith  of  Tay  to  the 
opposite  western  coast  of  the  Frith  of  Clyde.  In  Forfarshire, 
where,  as  in  Herefordshire,  it  is  many  thousand  feet  thick,  it 

*  Murchison's  Silurian  System,  p.  180.  t  Ibid.,  p.  183, 

t  See  Geol.  Trans.  2d  series,  vol.  in.  plates  15, 16, 17. 


PART  II.     CHAPTER  XXI. 


263 


Old  Red  Sandstone. 


may  be  divided  into  three  principal  masses  :  1st,  red  and  mottled 
marls,  cornstone  and  sandstone  ;  2d,  Conglomerate,  often  of  vast 
thickness ;  3d,  Tilestones  and  paving  stone,  highly  micaceous, 
and  containing  a  slight  admixture  of  carbonate  of  lime.  (See 
section,  p.  65.)  In  the  uppermost  of  these  divisions,  but  chiefly 
in  the  lowest,  the  remains  of  fish  have  been  found,  of  the  genus 
named  by  M.  Agassiz,  Cephalaspis,  or  tuckler-headed,  from  the 
extraordinary  shield  which  covers  the  head,  and  which  has  often 
been  mistaken  for  that  of  a  trilobite,  of  the  division  Asaphus. 
(See  Fig.  276.  p.  266.) 

Fig.  274. 


Cephalaspis  Lyellii,  Agass.    Length  6f  inches. 

This  figure  is  from  a  specimen  now  in  my  collection,  which  I  procured  at  Glam- 
mis,  in  Forfarshire ;  see  other  figures,  Agassiz,  vol.  ii.  Tab.  1.  a,  &  1.  6. 

a.  one  of  the  peculiar  scales  with  which  the  head  is  covered  when  perfect. — 
These  scales  are  generally  removed,  as  in  the  specimen  above  figured. 

b,  c.  scales  from  different  parts  of  the  body  and  tail. 

A  gigantic  species  of  fish  of  the  genus  Gyrolepis  has  also  been 
found  by  Dr.  Fleming  in  the  Old  Red  sandstone  of  Fifeshire.* 

*  See  Agassiz,  Poissons  Fossiles,  torn.  ii.  p.  139. 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Primary  Fossiliferous  Strata. 


CHAPTER  XXII. 

PRIMARY    FOSSILIFEROUS    STRATA. 

Primary  Fossiliferous  or  Transition  Strata — Terra  "Grauwacke" — Silurian 
Group — Upper  Silurian  and  Fossils — Lower  Silurian  and  Fossils — Trilobites — 
Graptolites — Orthocerata — Occasional  horizontality  of  Silurian  Strata — Cambrian 
Group — Endosiphonite. 

WE  have  now  arrived  in  the  descending  order  at  those  more 
ancient  sedimentary  rocks,  which  I  have  called  the  Primary 
Fossiliferous  (see  p.  157.),  and  to  which  Werner  first  gave  the 
name  of  Transition,  for  reasons  fully  explained  and  discussed  in 
the  12th  chapter.  Many  geologists  have  also  applied  to  these 
older  strata  the  general  name  of  "  grauwacke,"  by  which  the 
German  miners  designate  a  variety  of  quartzose  sandstone, 
which  is  usually  an  aggregate  of  small  fragments  of  quartz, 
flinty-slate  (or  Lydian  stone),  and  clay-slate,  cemented  together 
by  argillaceous  matter.  But  far  too  much  importance  has  been 
attached  to  this  kind  of  rock,  as  if  it  were  peculiar  to  a  certain 
epoch  in  the  earth's  history,  whereas  a  similar  sandstone  or  grit 
is  not  only  found  sometimes  in  the  Old  Red,  and  in  the  millstone 
grit  of  the  Coal,  and  in  certain  cretaceous  ^formations  of  the 
Alps — but  even  in  some  tertiary  deposits. 

In  England,  the  Old  Red  sandstone  has  been  generally  regard- 
ed as  the  base  of  the  secondary  series ;  but  by  some  writers  on 
the  Continent,  the  Old  Red  and  Coal  formations  have  been  class- 
ed as  the  upper  members  of  the  Transition  series,  a  method 
adopted  by  Dr.  Buckland,  in  his  late  Bridgewater  Treatise.  This 
classification,  however,  requires  us  to  draw  a  strong  line  of  de- 
marcation between  the  Coal  and  the  lower  New  Red  sandstone 
group,  which,  now  that  the  fossils  of  these  two  groups  are  ascer- 
tained to  be  very  analogous,  becomes  a  more  arbitrary  division 
than  that  which  separates  the  Old  Red  from  the  uppermost  of 
the  primary  fossiliferous  strata. 

Professor  Sedgwick  and  Mr.  Murchison  have  lately  proposed 
to  subdivide  all  the  English  sedimentary  strata  below  the  Old 
Red  sandstone  into  two  leading  groups,  the  upper  of  which  may 
be  termed  the  Silurian,  and  the  inferior  the  Cambrian  system. 
Mr.  Murchison  has  applied  the  name  of  Silurian  to  the  newer 
group,  because  these  rocks  may  be  best  studied  in  that  part  of 


PART  II.     CHAPTER  XXII. 


265 


Upper  Silurian  Rocks. 


England  and  Wales  which  was  included  in  the  ancient  British 
kingdom  of  the  Silures.  He  has  also  formed  four  subdivisions 
of  the  Silurian  system,  which  he  has  designated  as  the  Ludlow, 
Wenlock,  Caradoc,  and  Llandeilo,  indicating  thereby  the  places 
where  the  prevailing  characters  of  each  formation  are  most  per- 
fectly exhibited.  The  following  Table  explains  the  succession 
of  these  deposits.* 

UPPER    SILURIAN    ROCKS. 

Prevailing  Lithological        Thick- 
Characters,  ness. 


2000  ft.  < 


2.  Wenlock 
formation. 


formation. 


Upper 
Ludlow. 

>  Micaceous  grey" 
$  sandstone. 

1.  Ludlow       , 
formation.  ' 

Aymestry 
limestone. 

)  Argillaceous 
)  limestone. 

Lower 
Ludlow. 

C  Shale  with  con- 
<  cretions  of  lime- 
(  stone. 

Mollusca  marine,  of  al- 
most every  order,  the 
Brachiopoda  most 
abundant.  Serpula, 
Corals,  Sauroid  fish, 
Fuci. 


Wenlock    )  Concretionary 
limestone.  ( limestone. 


Wenlock 
shale. 


1  Argillaceous 
I  shale. 


f  Marine  mollusca  of  va- 
rious orders  as  before. 

}-1800ft.^  Crustaceans  of  the 
Trilobite  family. 

j  LNo  vertebrata  or  plants. 


LOWER  SILURIAN    ROCKS. 


Flags  of  shelly' 
limestone     and 


bedded      white 
^  freestone. 

)  Dark    coloured 
$  calcareous  flags. 


•  I  Crinoidea,  Corals,  Mol- 

2500  ft. «{      lusca,  chiefly  Brachio- 
poda, Trilobites. 

1200ft.  $  Mollusca,  Trilobites. 


UPPER  SILURIAN  ROCKS. 

Ludlow  formation. — This  member  of  the  upper  Silurian 
group,  as  will  be  seen  by  the  above  table,  is  of  great  thickness, 
and  subdivided  into  three  parts.  Each  of  these  may  be  distin- 
guished near  the  town  of  Ludlow,  and  at  other  places  in  Shrop- 
shire and  Herefordshire,  by  peculiar  organic  remains.  The 
most  remarkable  fossils  are  scales,  ichthyodorulites,  jaws,  teeth, 
and  coprolites  of  fish,  of  the  upper  Ludlow  rock.f  As  they 
are  the  oldest  remains  of  vertebrated  animals  yet  known  to  geo- 
logists, it  is  worthy  of  notice  that  they  belong  to  fish  of  a  high 
or  very  perfect  organization. 


See  Murchison's  Silurian  System. 
x 


t  Ibid.  p.  198,  199. 


266      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Upper  Silurian  Bocks,  and  their  Fossils. 

Among  the  fossil  shells  are  species  of  leptoena,  orthis,  terebra- 
tula,  avicula,  trochus,  orthoceras,  bellerophon,  and  others.* 

,    Fig.  275. 


Tcrebratula  Wilsoni,  Sow.    Ludlow  formation. 

Several  species  also  of  trilobite,  an  extinct  species  of  crusta- 
cean, characteristic  of  the  Silurian  period  in  general,  are  found  in 
the  lower  Ludlow  limestone.  Those  represented  in  the  annexed 
figures,  Calymene  Blumenbachii  and  Asaphus  caudatus,  are 
common  to  this  limestone,  and  to  the  Wenlock  formation  which 
succeeds  next  in  the  descending  order. 

Fig.  276. 


Calymene  Blumenbachii, 

Brongniart,  commonly  called 

"  Dudley  trilobite." 

Jlsaphus  oaudatus. 

Some  of  the  Upper  Ludlow  .sandstones  are  ripple-marked, 
thus  affording  evidence  of  gradual  deposition;  and  the  same 
may  be  said  of  the  fine  argillaceous  shales  of  the  Ludlow  form- 
ation, which  are  of  great  thickness,  and  have  been  provincially 
named  "  mudstones,"  from  their  tendency  to  dissolve  into  mud. 
In  these  shales  many  zoophytes  are  found  enveloped  in  an  erect 
position,  having  evidently  become  fossil  on  the  spots  where  they 
grew  at  the  bottom  of  the  sea.  Among  others,  the  graptolite  is 
abundant.  (See  p.  268.)  The  facility  with  which  these  upper 
Silurian  shales,  when  exposed  to  the  weather,  are  resolved  into 
mud,  proves  that,  notwithstanding  their  antiquity,  they  are  nearly 
in  the  state  in  which  they  were  first  thrown  down  at  the  bottom 
of  the  sea.  All  rocks,  therefore,  of  the  transition  era  of  Wer- 
ner, were  hot  originally  precipitated  in  a  semi-crystalline  state  as 
was  formerly  pretended.  (See  p.  153.) 


•*  Murchison,  Silurian  System,  p.  199. 


PART  II.     CHAPTER  XXII. 


267 


Lower  Silurian  Rocks,  and  their  Fossils. 


WenlocJc  formation.   The  well-known  Fig.  278. 

rock  of  Dudley,  so  rich  in  organic  re- 
mains, belongs  to  this  member  of  the 
upper  Silurian  group,  which  consists 
in  its  higher  division  of  limestone  more 
or  less  crystalline,  and  highly  charged 
with  corals  and  encrinites  of  species 
distinct  from  those  of  the  mountain 
limestone.  In  its  lower  part  it  is  prin- 
cipally composed  of  argillaceous  shale. 
In  the  Wenlock  limestone,  the  chain- 
coral,  called  Catenipora  cscharoidcs, 
abounds.  Among  the  shells  appear  the 
genera  euomphalus,  productus,  atrypa, 

and   many  Others.  Catenipora  escharoides. 


LOWER  SILURIAN  ROCKS. 

Caradoc  sandstone. — This  formation,  which  is  2500  feet 
thick,  consists  chiefly  of  sandstones  of  various  colours,  with 
some  subordinate  beds  of  calcareous  matter.  Almost  all  the 
more  abundant  fossils  belong  to  the  same  genera  as  those  of  the 
upper  Silurian  rocks,  but  the  species  are  distinct. 

Llandeilo  formation. — This  division,  forming  the  base  of  the 
Silurian  system,  consists  of  hard  dark-coloured  flags,  sometimes 
slightly  micaceous,  frequently  calcareous,  and  especially  distin- 
guished by  containing  the  large  trilobites 
Fig.  279.  Asaphus  Buchii  (Fig.  279.),  and  A.  ty- 

ranus.*  There  are  also  several  genera 
of  mollusca  in  this  deposit ;  and  it  is  an 
interesting  fact,  that  with  many  extinct 
forms  of  testacea  peculiar  to  the  lower 
Silurian  rocks,  such  as  orthoceras,  pen- 
temerus,  spirifer,  and  productus,  others 
JBucMi,  Brong.  are  associated  belonging  to  genera  still 

existing,  as  nautilus,  turbo,  buccinum,  turritella,  terebratula, 
and  orbicwla.'l" 

No  land  plants  seem  yet  to  have  been  discovered  in  strata 
which  can  be  unequivocally  demonstrated  to  belong  to  the  Silu- 
rian period. 

In  Norway  and  Sweden,  the  Silurian  strata  extend  over  a 
wide  area,  and  so  much  resemble  those  of  England  in  lithologi- 
cal  character  and  fossils,  that  they  will  probably  be  found  to  be 


Murchison,  Silurian  System,  p.  222. 


t  Ibid.  p.  351. 


268 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Lower  Silurian  Rocks,  and  their  Fossils. 


divisible  into  similar  groups.  They  are  composed  of  large  de- 
posits of  sandstone,  which  is  sometimes  found  at  the  base  of  the 
system,  resting  on  gneiss  and  calcareous  rocks,  with  orthocerata 
and  corals  ;  the  chain-coral,  (Fig.  278.)  before  mentioned,  being 
very  conspicuous ;  also  fine  bituminous  shales  containing  grap- 
tolites.  (Fig.  280.) 

These  bodies  are  supposed  by  Dr.  Beck,  of  Copenhagen,  to  be 
fossil  zoophytes,  related  to  the  family  of  sea-pens,  of  which  the 
living  animals  inhabit  mud  and  slimy  sediment. 

Fig.  280. 


Graptotttcs,  Linn. 

a.  6.  Graptolites  from  Christiania,  Norway. 
c.  d.  Graptolites  from  the  south  of  Sweden. 

In  the  limestones  of  Lake  Michigan,  in  North  America,  and 
other  regions  bordering  the  great  Canadian  lakes,  chain-corals 
and  trilobites  are  also  found,  and  from  their  fossils  generally 
they  seem  to  belong  to  the  Silurian  period.  They  contain  cer- 
tain orthocerata,  which  have  a  very  peculiar  structure.  The 
siphuncle  is  very  large,  and  has  a  tube  running  through  its 
whole  length,  from  the  outside  of  which  radii  set  off  in  verticila- 
tions  extending  to  the  inner  wall  of  the  syphon,  these  verticila- 
tions  corresponding  in  number  to  the  chambers  of  the  shell. 
Mr.  Stokes,  who  has  described  this  division  of  orthoceratites,  has 
formed  them  into  a  distinct 'genus,  for  which  he  has  adopted  the 

Fig.  281. 


Actinoceras  Simmsii,  Stokes. 
County  of  Down,  Ireland.    Length  of  original,  2  feet. 

name  of  Actinoceras,  proposed  by  Professor  Bronn.*     The  ac- 
tinoceras  was  not  known  as  a  British  fossil,  until  lately  disco- 

*  See  Proceedings,  Geol.  Soc.  1838. 


PART  II.     CHAPTER  XXII. 


Horizontal  Silurian  Strata. 


Fig.   282. 


A,  Simmsii,  portion  of  the  shell  at  a,  Fig.  281.,  natural  size, 
showing  the  tube  and  its  radii  within  the  siphuncle. 

vered  at  Castle  Espie,  in  the  county  of  Down,  in  Ireland.     (See 
Figs.  281,  282.) 

Silurian  strata  occasionally  horizontal. — The  Silurian  strata 
throughout  a  large  part  of  the  province  of  Skaraborg,  in  the 
south  of  Sweden,  are  perfectly  horizontal ;  the  different  subor- 
dinate formations  of  sandstone,  shale,  and  limestone,  occurring 
at  corresponding  heights  in  hills  many  leagues  distant  from  each 
other,  with  the  same  mineral  characters  and  organic  remains. 
It  is  clear  that  they  have  never  been  disturbed  since  the  time  of 
their  deposition,  except  by  such  gradual  movements  as  those  by 
which  large  areas  in  Sweden  and  Greenland  are  now  slowly  and 
insensibly  rising  above  or  sinking  below  their  former  level.  The 
ancient  limestone  and  shale;  also  of  the  Canadian  lake  district 
before  mentioned,  are  for  the  most  part  horizontal. 

These  facts  are  very  important,  as  the  more  ancient  rocks  are 
usually  much  disturbed,  and  horizontality  is  a  common  charac- 
ter of  newer  strata.  Similar  exceptions,  however,  occur  in  re- 
gard to  the  more  modern  or  tertiary  formations  which,  in  some 
places,  as  in  the  Alps,  are  not  only  vertical,  but  in  a  reversed 
position.  These  appearances  accord  best  with  the  theory  which 
teaches  that,  at  all  periods,  some  parts  of  the  earth's  crust  have 
been  convulsed  by  violent  movements,  which  have  been  some- 
times continued  so  long,  or  so  often  repeated,  that  the  derange- 
ment has  become  excessive,  while  other  spaces  have  escaped 
again  and  again,  and  have  never  once  been  visited  by  the  same 
kind  of  movement.  Had  paroxysmal  convulsions  ever  agitated 
simultaneously  the  entire  crust  of  the  earth,  as  some  have  ima- 
gined, the  primary  fossiliferous  strata  would  nowhere  have  re- 
mained horizontal. 
..* 


270      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Cambrian  Rocks. 

•  Cambrian  Group. — Below  the  Silurian  strata  in  the  region 
of  the  Cumberland  lakes,  in  N.  Wales,  Cornwall,  and  other 
parts  of  Great  Britain,  there  is  a  vast  thickness  of  stratified 
rocks,  for  the  most  part  slaty,  and  devoid  of  fossils.  In  some 
few  places  a  few  organic  remains  are  detected  specifically,  and 
some  of  them  generically,  distinct  from  those  of  the  Silurian 
period.  These  rocks  have  been  called  Cambrian  by  Professor 
Sedgwick,  because  they  are  largely  developed  in  N.  Wales, 
where  they  attain  a  thickness  of  several  thousand  yards.  They 
are  chiefly  formed  of  slaty  sandstone  and  conglomerate,  in  the 
midst  of  which  is  a  limestone  containing  shells  and  corals,  as  at 
Bala  in  Merionethshire.  A  slaty  sandstone,  forming  the  bottom 
of  the  Cambrian  system  in  Snowdon,  contains  shells  of  the 
family  Brachiopoda,  and  a  few  zoophytes.* 

In  some  of  the  slate  rocks  of  Cornwall,  referred  by  Professor 
Sedgwick  to  the  Cambrian  group,  cephalopoda  of  a  very  pecu- 
liar structure,  called  Endosiphonites,  have  been  detected,  a  form 
which  appears  not  yet  to  have  been  observed  in  the  Silurian 
formation.  The  siphuncle  in  this  shell  is  ventral,  in  which  cha- 
racter it  differs  both  from  ammonite,  in  which  it  is  dorsal,  and 
from  nautilus,  in  which  it  is  central,  or  nearly  central. 

Fig.  283. 


Endosiphonites  carinatus,  Ansted.f    Cambrian  strata,  Cornwall. 

Although  the  Cambrian  group  can  scarcely  yet  be  said  to  be 
established  on  the  evidence  of  a  distinct  assemblage  of  fossils, 
yet  so  great  is  the  thickness  of  strata  beneath  the  lowest  of  the 
well-determined  Silurian  rocks,  all  of  a  date  posterior  to  the 
creation  of  organic  beings,  that  we  may  reasonably  expect  to  be 
able  to  divide  the  primary  fossiliferous  strata  into  two  groups. 

*  Phillips's  Geology,  vol.  i.  p.  129.    Lafdner's  Cyclop,  vol.  xcvii. 
t  Camb.  Phil.  Trans,  vol.  vi.  pi.  8.  fig.  2. 


PART  II.     CHAPTER  X£III.  271 


Tests  of  Relative  Age  of  Volcanic  Rocks. 


CHAPTER  XXIII. 

ON   THE   DIFFERENT   AGES   OF   THE   VOLCANIC  ROCKS. 

Tests  of  relative  age  of  volcanic  rocks — Test  by  superposition  and  intrusion 
— By  alteration  of  rocks  in  contact — Test  by  organic  remains — Test  of  age  by 
mineral  character — Test  by  included  fragments — Volcanic  rocks  of  the  Recent 
and  Newer  Pliocene  periods — Miocene — Eocene — Cretaceous — Oolitic — New 
Red  sandstone  period — Carboniferous — Old  Red  sandstone  period — Silurian — 
Upper  and  lower  Cambrian  periods — Relative  ages  of  intrusive  traps. 

HAVING  referred  the  sedimentary  strata  to  a  long  succession 
of  geological  periods,  we  have  next  to  consider  how  far  the  vol- 
canic formations  can  be  classed  in  a  similar  chronological  order. 
The  tests  of  relative  age  in  this  class  of  rocks  are  four: — 1st, 
superposition  and  intrusion,  with  or  without  alteration  of  the 
rocks  in  contact ;  2d,  organic  remains ;  3d,  mineral  character  ; 
4th,  included  fragments  of  older  rocks. 

Test  by  superposition,  fyc. — If  a  volcanic  rock  rest  upon  an 
aqueous  deposit,  the  former  must  be  the  newest  of  the  two,  but 
the  like  rule  does  not  hold  good  where  the  aqueous  formation 
rests  upon  the  volcanic,  for  we  have  already  seen  (p.  110.)  that 
melted  matter,  rising  from  below,  may  penetrate  a  sedimentary 
mass  without  reaching  the  surface,  or  may  be  forced  in  con- 
formably between  two  strata,  as  b  at  D  in  the  annexed  figure 
(Fig.  284.)  after  which  it  may  cool  down  and  consolidate.  Su- 

Fig.  284. 


^—  ^____ 

D 

iiziziSI^S 

',','  i  ,«.  '  ,'•  '  •  '£.'  i  ' 

,','•'  i1  '  i 

c 

c 

perposition,  therefore,  is  not  of  the  same  value  as  a  test  of  age 
in  the  unstratified  volcanic  rocks  as  in  fossiliferous  formations. 
We  can  only  rely  implicitly  on  this  test  where  the  volcanic 
rocks  are  contemporaneous,  not  where  they  are  intrusive.  Now 
they  are  said  to  be  contemporaneous  if  produced  by  volcanic 
action,  which  was  going  on  simultaneously  with  the  deposition 
of  the  strata  with  which  they  are  associated.  Thus  in  the  sec- 


272 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Tests  of  Relative  Age  of  Volcanic  Rocks 


tion  at  D  (Fig.  284.),  wre  may  perhaps  ascertain  that  the  trap  b 
flowed  over  the  fossiliierous  bed  c,  and  that,  after  its  consolida- 
tion, a  was  deposited  upon  it,  a  and  c  both  belonging  to  the  same 
geological  period.  But  if  the  stratum  a  be  altered  by  b  at  the 
point  of  contact,  we  must  then  conclude  the  trap  to  have  been 
intrusive,  or  if,  in  pursuing  b  for  some  distance,  we  find  at  length 
that  it  cuts  through  the  stratum  a,  and  then  overlies  it. 

We  may,  however,  be  easily  deceived  in  supposing  a  volcanic 
rock  to  be  intrusive,  when  in  reality  it  is  contemporaneous,  for  a 
sheet  of  lava,  as  it  spreads  over  the  bottom  of  the  sea,  cannot 
rest  every  where  upon  the  same  stratum,  either  because  these 
have  been  denuded,  or  because,  if  newly  thrown  down,  they  thin 
out  in  certain  places,  thus  allowing  the  lava  to  cross  their  edges. 
Besides,  the  heavy  igneous  fluid  will  often,  as  it  moves  along, 
cut  a  channel  into  beds  of  soft  mud  and  sand.  Suppose  the  sub- 
marine lava  F,  to  have  come  in  contact  in  this  manner  with  the 

strata  a,  &,  c,  and  that,  after  its 
Fig.  285.  consolidation,  the  strata  d,  e,  are 

thrown  down  in  a  nearly  horizon- 
tal position,  yet  so  as  to  lie  uncon- 
formably  to  F,  the  appearance  of 
subsequent  intrusion  will  here  be 
complete,  although  the  trap  is  in 
fact  contemporaneous.  We  must, 
unless  we  find  the  strata  d  or  e  to 
have  been  altered  at  thoir  junction,  as  if  by  heat,  not  therefore 
hastily  infer  that  the  rock  F  is  intrusive. 

When  trap  dikes  were  described  in  the  8th  chapter,  they  were 
shown  to  be  more  modern  than  all  the  strata  which  they  tra- 
verse. The  ninety-fathom  dike  in  the  Northumberland  coal- 
field (see  section,  Fig.  286.)  passes  through  coal-measures  which 


Magnesian  limestone. 


Coal. 


Dike. 
Section  in  a  quarry  at  Cullercoats,  Northumberland. 

are  much  disturbed.*     The  beds  of  overlying  Magnesian  lime- 
stone are  not  cut  through  by  the  dike,  but  appear  to  be  in  the 


*  See  Mr.  Winch's  account,  Geol.  Trans.  1st  ser.  vol.  iv.  p.  1. 


PART  II.     CHAPTER  XXIII.  273 


Tests  of  Relative  Age  of  Volcanic  Rocks. 


position  in  which  they  were  originally  deposited  in  a  hollow, 
existing  in  the  denuded  surface  formed  by  the  carboniferous 
strata  and  intrusive  dike.  Now  here  the  coal-measures  were 
not  only  deposited,  but  had  been  fissured  before  the  fluid  trap 
was  introduced  to  form  the  dike.  It  also  appears  by  the  trun- 
cated edges  of  the  Coal  strata,  and  the  abrupt  termination  of  the 
dike  on  which  the  Magnesian  limestone  rests,  that  denudation 
had  taken  place  at  a  period  intervening  between  the  injection  of 
the  volcanic  matter  and  the  deposition  of  the  Magnesian  lime- 
stone. Even  in  this  case,  however,  although  the  date  of  the  vol- 
canic eruption  is  brought  within  Harrow  limits,  it  cannot  be 
defined  with  precision ;  it  may  have  happened  either  at  the  close 
of  the  carboniferous  period,  or  early  in  that  of  the  lower  New 
Red  sandstone,  or  between  these  two  periods,  when  the  state  of 
the  animate  creation,  and  the  physical  geography  of  Europe 
were  gradually  changing  from  the  type  of  the  carboniferous  era 
to  that  of  the  lower  New  Red  formation. 

In  regard  to  all  stratified  volcanic  tuffs,  the  test  of  age  by 
superposition  is  strictly  applicable  to  them,  according  to  the 
already  explained  rules  in  the  case  of  other  sedimentary  deposits 
(See  p.  159.) 

Test  of  age  by  organic  remains. — We  have  seen  how,  in  the 
vicinity  of  active  volcanos,  scoriae,  pumice,  fine  sand,  and  frag- 
ments of  rock  are  thrown  up  into  the  air,  and  then  showered 
down  upon  the  land,  or  into  neighbouring  lakes  or  seas.  In  the 
tuffs  so  formed,  shells,  corals,  or  any  other  durable  organic 
bodies  which  may  happen  to  be  strewed  over  the  bottom  of  a 
lake  or  sea,  will  be  imbedded  in  tuff,  and  thus  continue  as  per- 
manent memorials  of  the  geological  period  when  the  volcanic 
eruption  occurred.  Tufaceous  strata  thus  formed  in  the  neigh- 
bourhood of  Vesuvius,  Etna,  Stromboli,  and  other  volcanos  now 
active  in  islands  or  near  the  sea,  may  give  information  of  the 
relative  age  of  these  tuffs  at  some  remote..future  period  when  the 
fires  of  these  mountains  are  extinguished.  By  such  evidence  we 
can  distinctly  establish  the  coincidence  in  age  of  volcanic  rocks, 
and  the  different  primary,  secondary,  and  tertiary  fossiliferous 
strata  already  considered. 

The  tuffs  now  alluded  to  are  not  exclusively  marine,  but 
include,  in  some  places,  freshwater  shells,  in  others,  the  bones 
of  terrestrial  quadrupeds.  The  diversity  of  organic  remains  in 
formations  of  this  nature  is  perfectly  intelligible,  if  we  reflect 
on  the  wide  dispersion  of  ejected  matter  during  late  eruptions, 
such  as  that  of  the  volcano  of  Coseguina,  in  the  province  of 
Nicaragua,  January  19,  1835.  Hot  cinders  and  fine  scoriae 
were  then  cast  up  to  a  vast  height,  and  covered  the  ground  as 


274  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Tests  of  Relative  Age  of  Volcanic  Rocks. 

they  fell  to  the  depth  of  more  than  ten  feet,  and  for  a  distance 
of  eight  leagues-from  the  crater  in  a  southerly  direction.  Birds, 
cattle,  and  wild  animals  were  scorched  to  death  in  great  num- 
bers, and  buried  in  these  ashes.  Some  volcanic  dust  fell  at 
Chiapa,  upwards  of  1200  miles  to  windward  of  the  volcano,  a 
striking  proof  of  a  counter-current  in  the  upper  region  of  the 
atmosphere,  and  some  on  Jamaica,  about  700  miles  distant  to 
the  north-east.  In  the  sea  also,  at  the  distance  of  1100  miles 
from  the  point  of  eruption,  Captain  Eden  of  the  Conway  sailed 
40  miles  through  floating  pumice,  among  which  were  some  pieces 
of  considerable  size.* 

Test  of  age  by  mineral  -composition. — As  sediment  of  homo- 
geneous composition,  when  discharged  from  the  mouth  of  a  large 
river,  is  often  deposited  simultaneously  over  a  wide  space,  so  a 
particular  kind  of  lava  flowing  from  a  crater,  during  one  erup- 
tion, may  spread  over  an  extensive  area,  as  in  Iceland,  in  1783, 
when  the  melted  matter,  pouring  from  Skaptar  Jokul,  flowed  in 
streams  in  opposite  directions,  and  caused  a  continuous  mass, 
the  extreme  points  of  which  were  90  miles  distant  from  each 
other.  This  enormous  current  of  lava  varied  in  thickness  from 
100  feet  to  600  feet,  and  in  breadth,  from  that  of  a  narrow  river 
gorge  to  15  miles.f  Now,  if  such  a  mass  should  afterwards  be 
divided  into  separate  fragments  by  denudation,  we  might  still 
perhaps  identify  the  detached  portions  by  their  similarity  in 
mineral  composition.  Nevertheless,  this  test  will  not  always 
avail  the  geologist,  for,  although  there  is  usually  a  prevailing 
character  in  lava  emitted  during  the  same  eruption,  and  even  in 
the  successive  currents  flowing  from  the  same  volcano,  still,  in 
many  cases,  the  different  parts  even  of  one  lava-stream,  or,  as 
before  stated,  of  one  continuous  mass  of  trap,  vary  so  much  in 
mineral  composition  and  texture,  as  to  render  these  characters 
of  minor  importance  when  compared  to  their  value  in  the  chro- 
nology of  the  fossiliferous  rocks. 

It  will,  however,  be  seen  in  the  description  which  follows,  of 
the  European  trap  rocks  of  different  ages,  that  they  had  often  a 
peculiar  lithological  character,  resembling  the  differences  before 
remarked  as  existing  between  the  modern  lavas  of  Vesuvius, 
Etna,  and  Chili.  (See  p.  98.) 

It  has  been  remarked  that  in  Auvergne,  the  Eifel,  and  other 
countries  where  trachyte  and  basalt  are  both  present,  the  trachytic 
rocks  are  for  the  most  part  older  than  the  basaltic.  These  rocks  do, 
indeed,  sometimes  alternate  partially,  as  in  the  volcano  of  Mount 

*  Caldcleugh,.Phil.  Trans.  1836,  p.  27.,  and  Official  Documents  of  Nicaragua, 
t  See  Principles  of  Geology,  Index,  "  Skaptar  Jokul." 


PART  II.     CHAPTER  XXIII.  275 


Different  Ages  of  Volcanic  Rocks. 


Dor,  in  Auvergne ;  but  the  great  mass  of  trachyte  occupies  in 
general  an  inferior  position,  and  is  cut  through  arid  overflowed 
by  basalt.  It  can  by  no  means  be  inferred  that  trachyte  pre- 
dominated greatly  at  one  period  of  the  earth's  history,  and  basalt 
at  another,  for  we  know  that  trachytic  lavas  have  been  formed 
at  many  successive  periods,  and  are  still  emitted  from  many 
active  craters ;  but  it  seems  that  in  each  region,  where  a  long 
series  of  eruptions  have  occurred,  the  more  felspathic  lavas  have 
been  first  emitted,  and  the  escape  of  the  more  augitic  kinds  has 
followed.  The  hypothesis  suggested  by  Mr.  Scrope  may,  per- 
haps, afford  a  solution  of  this  problem.  The  minerals,  he  ob- 
serves, which  abound  in  basalt  are  of  greater  specific  gravity 
than  those  composing  the  felspathic  lavas ;  thus,  for  example, 
hornblende,  augite,  and  olivine,  are  each  more  than  three  times 
the  weight  of  water ;  whereas  common  felspar,  albite,  and  La- 
brador felspar,  have  each  scarcely  more  than  "2\  times  the  specific 
gravity  of  water ;  and  the  difference  is  increased  in  Consequence 
of  there  being  much  more  iron  in  a  metallic  state  in  basalt  and 
greenstone  than  in  trachyte  and  other  felspathic  lavas  and  traps. 
If,  therefore,  a  large  quantity  of  rock  be  melted  up  in  the  bowels 
of  the  earth  by  volcanic  heat,  the  denser  ingredients  of  the  boil- 
ing fluid  will  sink  to  the  bottom,  and  the  lighter  remaining  above 
will  be  first  propelled-  upwards  to  the  surface  by  the  expansive 
power  of  gases.  Those  materials,  therefore,  which  occupied 
the  lowest  place  in  the  subterranean  reservoir  will  always  be 
emitted  last,  and  take  the  uppermost,  place  on  the  exterior  of  the 
earth's  crust. 

Test  by  included  fragments. — We  may  sometimes  discover 
the  relative  age  of  two  trap  rocks,  or  of  an  aqueous  deposit  and 
the  trap  on  which  it  rests,  by  finding  fragments  of  one  included 
in  the  other,  in  cases  such  as  those  before  alluded  to,  where  the 
evidence  of  superposition  alone  would  be  insufficient.  It  is  also 
not  uncommon  to  find  conglomerates  almost  exclusively  com- 
posed of  rolled  pebbles  of  trap,  associated  with  stratified  rocks  in 
the  neighbourhood  of  masses  of  intrusive  trap.  If  the  pebbles 
agree  generally  in  mineral  character  with  the  latter,  we  are  then 
enabled  to  determine  the  age  of  the  intrusive  rock  by  knowing 
that  of  the  fossiliferous  strata  associated  with  the  conglomerate. 
The  origin  of  such  conglomerates  is  explained  by  observing  the 
shingle  beaches  composed  of  trap  pebbles  in  modern  volcanic 
islands,  or  at  the  base  of  Etna. 

Recent  and  newer  Pliocene  period. — J  shall  now  select  ex- 
amples of  contemporaneous  volcanic  rocks  of  successive  geolo- 
gical periods,  that  the  reader  may  be  convinced  that  the  igneous 
causes  have  been  in  activity  in  all  past  ages  of  the  world,  and 


276      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Different  Ages  of  Volcanic  Rocks. 

that  they  have  been  ever  shifting  the  places  where  they  have 
broken  out  at  the  earth's  surface.  One  portion  of  the  lavas,  tuffs, 
and  trap-dikes  of  Etna,  Vesuvius,  and  the  island  of  Ischia,  have 
been  produced  within  the  historical  era ;  another  and  a  far  more 
considerable  part  have  originated  at  times  immediately  antece- 
dent, when  the  waters  of  the  Mediterranean  were  already  inha- 
bited by  the  existing  species  of  testacea.  The  submarine  foun- 
dations of  Etna  and  Ischia  have  been  upheaved  to  the  great 
height  of  between  500  and  1500  feet  above  the  level  of  the  sea ; 
and  the  same  observations  may  be  made  respecting  the  base  of 
many  active  volcanos  which  were  first  subaqueous  vents,  or,  like 
Stromboli,  half  submerged,  and  then  became  subaerial,  when  the 
ancient  bed  of  the  sea  was  laid  dry  by  elevation. 

Older  Pliocene  period. — In  Tuscany  and  the  Campagna  di 
Roma,  submarine  volcanic  tuffs  are  interstratified  with  the  Older 
Pliocene  strata  of  the  Subapennine  hills  in  such  a  manner  as  to 
leave  no  doubt  that  they  were  the  products  of  eruptions  which 
occurred  wnen  the  shelly  marls  and  sands  of  the  Subapennine 
hills  were  in  the  course  of  deposition. 

Miocene  period. — The  most  ancient  volcanic  rocks,  consisting 
chiefly  of  trachyte,  of  the  Upper  and  Lower  Eifel,  are  interca- 
lated between  Miocene  strata  in  such  a  manner,  as  to  prove 
them  to  have  been  coeval  in  origin.  The  eruptions,  however, 
of  the  same  district  were  continued  down  to  the  Newer  Pliocene 
era,  or  were  at  least  renewed  at  that  later  period,  so  that  show- 
ers of  ashes  from  the  Rhenish  volcanos  are  interstratified  with 
the  loess,  in  which,  we  have  already  stated,  shells  of  land  and 
freshwater  species  occur  identical  with  those  now  living  in 
Europe.* 

Eocene  period. — The  extinct  volcanos  of  Auvergne  and  Can- 
tal,  in  central  France,  commenced  their  eruptions  in  the  Eocene 
period,  but  were  most  active  during  the  Miocene  era.  In  the 
lacustrine  deposits,  near  those  ancient  volcanos,  the  lowest  strata 
were  evidently  formed  before  any  eruptions  had  occurred.  They 
consist  of  sandstone  and  conglomerate,  containing  rounded  peb- 
bles of  quartz,  mica-schist,  granite,  and  other  hypogene  rocks, 
composing  the  borders  of  the  ancient  lakes,  but  not  the  slightest 
intermixture  of  volcanic  products  can  be  detected.  To  these 
conglomerates  succeeded  argillaceous  and  calcareous  marls, 
containing  Eocene  shells,  during  the  deposition  of  which  some 
feeble  signs  of  volcanic  action  began  to  show  themselves. 
Above  these,  freshwater  marls  and  limestones  are  seen  frequently 
to  alternate  with  volcanic  tuff,  and  in  them  some  fossils  of  the 

*See  above,  p.  172.,  and  Principles  of  Geology,  book  iv. 


PART  II.     CHAPTER  XXIII.  277 

Volcanic  Rocks  of  the  Cretaceous  Period. 

Miocene  period  are  discovered.  After  the  filling  up  or  drainage 
of  the  ancient  lakes,  huge  piles  of  trachytic  and  basaltic  rocks, 
with  volcanic  breccias  and  conglomerates,  accumulated  to  a 
thickness  of  several  thousand  feet,  and  were  superimposed  upon 
granite,  or  the  contiguous  lacustrine  strata.  The  greater  portion 
of  these  igneous  rocks  appear  to  have  originated  during  the  Mio- 
cene period,  and  extinct  quadrupeds  of  that  era,  belonging  to  the 
genera  Mastodon,  Rhinoceros,  and  others,  were  buried  in  ashes 
and  beds  of  alluvial  sand  and  gravel,  which  owe  their  preserva- 
tion to  sheets  of  lava  which  spread  over  them. 

Cretaceous  period. — Although  we  have  no  proof  of  volcanic 
rocks  erupted  in  England  during  the  deposition  of  the  chalk  and 
green-sand,  it  must  not  be  supposed  that  no  theatres  of  igneous 
action  existed  in  the  cretaceous  period.  M.  Virlet,  in  his  account 
of  the  geology  of  the  Morea,*  (p.  205.)  has  clearly  shown  that 
certain  traps  in  Greece,  called  by  him  ophiolites,  are  of  this  date; 
as  those,  for  example,  which  alternate  conformably  with  creta- 
ceous limestone  and  green-sand  between  Kastri  and  Damala  in 
the  Morea.  They  consist  in  great  part  of  dial! age  rocks  and 
serpentine,  and  of  an  amygdaloid  with  calcareous  kernels,  and  a 
base  of  serpentine. 

In  certain  parts  of  the  Morea,  the  age  of  these  volcanic  rocks 
is  established  by  the  following  proofs :  first,  the  lithographic 
limestones  (see  p.  199.)  of  the  cretaceous  era  are  cut  through  by 
trap,  and  then  a  conglomerate  occurs  at  Nauplia  and  other  places, 
containing  in  its  calcareous  cement  many  well-known  fossils  of 
the  chalk  and  green-sand,  together  with  pebbles  formed  of  rolled 
pieces  of  the  same  ophiolite,  which  appear  in  the  dikes  above 
alluded  to. 

It  was  before  stated  that  at  Tercis,  near  Dax,  in  the  depart- 
ment of  the  Landes,  in  the  south  of  France,  highly  inclined 

Fig.  287.     AdourR.  Luy  R.     Puy  Arzet. 

' 


Chalk  and  volcanic  tuff  in  the  environs  of  Dax. 
E  Inclined  beds  of  chalk,  and  conformable  volcanic  tuff 
a.  b.  c.  d.  Gravel,  sand,  and  tertiary  strata. 

strata  of  limestone  and  marl  occur,  containing  the  fossils  of  the 
chalk,  the  inclined  strata  being  in  great  part  concealed  by 
unconformable  tertiary  formations.  In  one  section  in  this  dis- 
trict I  observed,  alternating  with  thin  layers  of  volcanic  tuff, 
vertical  cretaceous  beds,  which  are  perfectly  conformable.  Such 


278  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Volcanic  Rocks. 

tuffs  were  probably  the  product  of  submarine  eruptions  in  the 
cretaceous  sea. 

The  traps  of  this  country  and  of  the  neighbouring  Pyrenees 
are  generally  ophitic,  and  many  French  geologists  consider  them 
to  be  newer  than  the  cretaceous  period,  and  therefore  tertiary ; 
but  I  know  of  no  sections  which  demonstrate  this  point.  M. 
Charpentier  has  argued  that  the  ophites  of  the  Pyrenees  were 
more  modern  than  all  the  secondary  strata  of  that  chain,  because 
in  the  conglomerates  constituting  the  upper  part  of  the  creta- 
ceous series  on  the  flanks  of  the  Pyrenees,  no  rolled  pebbles  of 
ophite  have  been  found.*  But  this  negative  fact  may  be  ex- 
plained by  supposing  that,  in  the  cretaceous  sea,  which  occupied 
the  space  where  the  Pyrenees  now  stand,  the  ophitic  eruptions 
were  submarine,  and  never  formed  islands  or  shoals  exposed  to 
denudation. 

The  age  of  the  trap  of  Antrim  in  Ireland,  before  described,  as 
altering  the  chalk  by  its  dikes  (p.  107.),  is  uncertain.  It  is 
newer  than  the  chalk  of  that  region,  which  it  cuts  through  and 
overflows ;  and  perhaps  it  belongs  to  some  one  of  the  tertiary 
periods.  As  wood-coal  and  coniferous  fossil  trees  have  been 
found  associated  with  it  on  the  eastern  shore  of  Lough  Neagh, 
these  plants  may  hereafter  throw  light  on  this  chronological 
question,  f 

Period  of  Oolite  and  Lias. — Although  the  green  and  serpen- 
tinous  trap  rocks  of  the  Morea  belong  chiefly  to  the  cretaceous 
era,  as  before-mentioned,  yet  it  seems  that  some  eruptions  of 
similar  rocks  began  during  the  oolitic  period  ;f  and  it  is  proba- 
ble, that  a  large  part  of  the  trappean  masses,  called  ophiolites  in 
the  Apennines,  and  associated  with  the  limestone  of  that  chain, 
are  of  corresponding  age. 

Whether  part  of  the  volcanic  rocks  of  the  Hebrides,  in  our 
own  country,  originated  contemporaneously  with  the  lias  and 
oolite  which  they  traverse  and  overlie,  remains  to  be  ascer- 
tained. 

Trap  of  the  New  Red  sandstone  period. — In  the  southern 
part  of  Devonshire,  trappean  rocks  are  associated  with  new  red 
sandstone,  and,  according  to  Mr.  De  la  Beche,  have  not  been 
intruded  subsequently  into  the  sandstone,  but  were  produced  by 
contemporaneous  volcanic  action.  Some  beds  of  grit,  mingled 
with  ordinary  red  marl,  resemble  sands  ejected  from  a  crater; 
and  in  the  stratified  conglomerates  occurring  near  Tiverton  are 

*  Charpentier,  Essai  Geog.  sur  ies  Pyrenees,  p.  524. 
t  Dr.  Berger,  Geol.  Trans.  1st  series,  vol.  iii.  p.  183. 
t  Boblaye  and  Virlet,  Morea,  p.  23. 


PART  II.     CHAPTER  XXIII.  279 


Trap  coeval  with  Coal  and  Old  Red  Sandstone. 


many  angular  fragments  of  trap  porphyry,  some  of  them  one  or 
two  tons  in  weight,  intermingled  with  pebbles  of  other  rocks. 
These  angular  fragments  were  probably  thrown  out  from  vol- 
canic vents,  and  fell  upon  sedimentary  matter,  then  in  the  course 
of^deposition.* 

Carboniferous  period.— -Two  classes  of  contemporaneous  trap 
rocks  have  been  ascertained  by  Dr.  Fleming  to  occur  in  the  coal- 
field of  the  Forth  in  Scotland.  The  newest  of  these,  connected 
with  the  higher  series  of  coal-measures,  is  well  exhibited  along 
the  shores  of  the  Forth,  in  Fifeshire,  where  they  consist  of  basalt 
with  olivine,  amygdaloid,  greenstone,  wacke,  and  tuff.  They 
appear  to  have  been  erupted  while  the  sedimentary  strata  were 
in  a  horizontal  position,  and  to  have  suffered  the  same  disloca- 
tions which  those  strata  have  subsequently  undergone.  In  the 
volcanic  tuffs  of  this  age  are  found  not  only  fragments  of  lime- 
stone, shale,  flinty  slate,  and  sandstone,  but  also  pieces  of  coal. 

The  other  or  older  class  of  carboniferous  traps  are  traced 
along  the  south  margin  of  Stratheden,  and  constitute  a  ridge 
parallel  with  the  Ochils,  and  extending  from  Stirling  to  near  St. 
Andrew's.  They  consist  almost  exclusively  of  greenstone,  be- 
coming, in  a  few  instances,  earthy  and  amygdaloidal.  They 
are  regularly  interstratified  with  the  sandstone,  shale,  and  iron- 
stone of  the  lower  coal-measures,  and,  on  the  East  Lomond, 
with  Mountain  limestone,  f 

Trap  of  the  Old  Red  sandstone  period. — By  referring  to  the 
section  explanatory  of  the  structure  of  Forfarshire,  already  given 
(p.  65.),  the  reader  will  perceive  that  the  beds  of  conglomerate, 
No.  3.,  occur  in  the  middle  of  the  old  red  sandstone  system,  1, 
2,  3,  4.  The  pebbles  in  these  conglomerates  are  sometimes 
composed  of  granitic  and  quartz  rocks,  sometimes  exclusively 
of  different  varieties  of  trap,  which,  although  purposely  omitted 
in  the  above  section,  are  often  found,  either  intruding  themselves 
in  amorphous  masses  and  dikes  into  the  older  fossiliferous  tile- 
stones,  No.  4.,  or  alternating  with  them  in  conformable  beds. 
All  the  different  divisions  of  the  red  sandstone,  1,  2,  3,  4,  are 
occasionally  intersected  by  dikes,  but  they  are  very  rare  in  Nos. 
1.  and  2.,  the  upper  members  of  the  group  consisting  of  red 
shale  and  red  sandstone.  These  phenomena,  which  occur  at  the 
foot  of  the  Grampians,  are  repeated  in  the  Sidlaw  Hills ;  and  it 
appears  that  in  this  part  of  Scotland  volcanic  eruptions  were 
most  frequent  in  the  earlier  part  of  the  old  red  sandstone  period. 

The  trap  rocks  alluded  to  consist  chiefly  of  felspathic  porphy- 

*De  la  Beche,  Geol.  Proceedings,  No.  41.  p.  196. 

t  Fleming  MS.    Part  of  this  tract  I  have  myself  examined  with  Dr.  F. 


280      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Silurian  Volcanic  Rocks Cambrian  Volcanic  Rocks. 

ry  and  amygdaloid,  the  kernels  of  the  latter  being  sometimes 
calcareous,  often  chalcedonic,  and  forming  beautiful  agates.  We 
meet  also  with  claystone,  clinkstone,  greenstone,  compact  felspar, 
and  tuff.  Some  of  these  rocks  flowed  as  lavas  over  the  bottom 
of  the  sea,  and  enveloped  quartz  pebbles  which  were  lying  there, 
so  as  to  form  conglomerates  with  a  base  of  greenstone,  as  is 
seen  in  Lumley  Den,  in  the  Sidlaw  Hills.  On  either  side  of 
the  axis  of  this  chain  of  hills  (see  section,  p.  65.),  the  beds  of 
massive  trap,  and  the  tuffs  composed  of  volcanic  sand  and  ashes, 
dip  regularly  to  the  south-east  or  north-west,  conformably  with 
the  shales  and  sandstones. 

Dr.  Fleming  has  observed  similar .  trap  rocks  in  the  old  red 
sandstone  of  northern  Fifeshire,  where  they  are  covered  imme- 
diately by  the  yellow  sandstone  which  forms  the  base  of  the 
mountain  limestone  and  coal-measures. 

Silurian  period. — It  appears  from  the  investigations  of  Mr. 
Murchison  in  Shropshire,  that  when  the  lower  Silurian  strata  of 
that  county  were  accumulating,  there  were  frequent,  volcanic 
eruptions  beneath  the  sea ;  and  the  ashes  and  scoriae  then  ejected, 
gave  rise  to  a  peculiar  kind  of  tufaceous  sandstone  or  grit,  dis- 
similar to  the  other  rocks  of  the  Silurian  series,  and  only  ob- 
servable in  places  where  syenitic  and  other  trap  rocks  protrude.* 
These  tuffs  occur  on  the  flanks  of  the  Wrekin  and  Caer  Cara- 
doc,  and  contain  Silurian  fossils,  such  as  casts  of  encrinites,  tri- 
lobites  and  mollusca.  Although  fossil iferous,  the  stone  resembles 
a  sandy  claystone  of  the  trap  family.f 

Thin  layers  of  trap,  only  a  few  inches  thick,  alternate,  in 
some  parts  of  Shropshire  and  Montgomeryshire,  with  sedimen- 
tary strata  of  the  lower  Silurian  system.  This  trap  consists  of 
slaty  porphyry  and  granular  felspar  rock,  the  beds  being  tra- 
versed by  joints  like  those  in  the  associated  sandstone,  limestone, 
and  shale,  and  having  the  same  strike  and  dip.J 

In  Radnorshire,  there  is  an  example  of  twelve  bands  of  stra- 
tified trap  alternating  with  Silurian  schists  and  flagstones  in  a 
thickness  of  350  feet.  The  bedded  traps  consist  of  felspar-por- 
phyry, clinkstone,  and  other  varieties  ;  and  the  interposed  Llan- 
deilo  flags  are  of  sandstone  and  shale,  with  trilobites  and  grap- 
tolites.§ 

Cambrian  volcanic  rocks.  —  In  Pembrokeshire  stratified, 
greenstone,  felspar-rock,  and  a  breccia  containing  fragments  of 
trap,  alternate  conformably  in  thick  parallel  masses  with  regu- 
larly stratified  sandstone  and  schist  of  the  upper  Cambrian  sys- 

*  Murchison,  Silurian  System,  &c.  p.  230. 

t  Ibid.  J  Ibid.  p.  272.  $  Ibid.  p.  325. 


PART  II.     CHAPTER  XXIV.  281 

Tests  of  Relative  Age  of  Plutonic  Rocks. 

tern.  These  trappean  masses,  says  Mr.  Murchison,  must  have 
been  evolved  at  intervals  from  volcanic  fissures  at  the  bottom  of 
the  sea,  when  the  sand,  pebbles,  and  mud,  now  forming  the  ac- 
companying sedimentary  rocks,  were  deposited.* 

Professor  Sedgwick,  in  his  account  of  the  geology  of  Cum- 
berland, has  described  various  trap-rocks  which  accompany  the 
green  slates  of  the  Cambrian  system,  beneath  a  limestone  con- 
taining organic  remains.  Different  felspathic  and  porphyritic 
rocks  and  greenstones  occur,  not  only  in  dikes,  but  in  conform- 
able beds  ;  and  there  is  occasionally  a  passage  from  these  igne- 
ous rocks  to  some  of  the  green  quartzose  slates.  Professor 
Sedgwick  supposes  these  porphyries  to  have  originated  contem- 
poraneously with  the  stratified  chloritic  slates,  the  materials  of 
the  slates  having  been  supplied,  in  part  at  least,  by  submarine 
eruptions  oftentimes  repeated. f 


CHAPTER  XXIV. 


ON  THE   DIFFERENT   AGES  OF  THE  PLUTONIC  ROCKS. 

Difficulty  in  ascertaining  the  precise  age  of  a  plutonic  rock— Test  of  age  by 
relative  position — Test  by  intrusion  and  alteration — Test  by  mineral  composition 
— Test  by  included  fragments — Recent  and  Pliocene  plutonic  rocks,  why  invisi- 
ble— Tertiary  plutonic  rocks  in  the  Andes — Granite  altering  Cretaceous  rocks — 
Granite  altering  Lias  in  the  Alps  and  in  Sky — Granite  of  Dartmoor  altering  Car- 
boniferous strata — Granite  of  the  Old  Red  sandstone  period — Syenite  altering 
Silurian  strata  in  Norway — Blending  of  the  same  with  gneiss — Most  ancient 
plutonic  rocks — Granite  protruded  in  a  solid  form — On  the  probable  age  of  the 
granite  of  Arran,  in  Scotland. 

WHEN  we  adopt  the  igneous  theory  of  granite,  as  explained 
in  the  9th  chapter,  and  believe  that  different  plutonic  rocks  have 
originated  at  successive  periods  beneath  the  surface  of  the  planet, 
we  must  be  prepared  to  encounter  greater  difficulty  in  ascertain- 
ing  the  precise  age  of  such  rocks,  than  in  the  case  of  volcanic 
and  fossiliferous  formations.  We  must  bear  in  mind,  that  the 
evidence  of  the  age  of  each  contemporaneous  volcanic  rock  was 
derived,  either  from  lavas  poured  out  upon  the  ancient  surface, 

*  Murchison,  Silurian  System,  &c.  p.  404. 
t  Geol.  Trans.  2d  series,  vol.  iv.  p.  55. 


282      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Tests  of  Relative  Age  of  Plutonic  Rocks. 

whether  in  the  sea  or  in  the  atmosphere,  or  from  tuffs  and  con- 
glomerates, also  deposited  at  the  surface,  and  either  containing 
organic  remains  themselves,  or  intercalated  between  strata  con- 
taining fossils.  But  all  these  tests  fail  when  we  endeavour  to  fix 
the  chronology  of  a  rock,  which  has  crystallized  from  a  state  of 
fusion  in  the  bowels  of  the  earth.  In  that  case,  we  are  reduced 
to  the  following  tests ;  1st,  relative  position  ;  2dly,  intrusion  and 
alteration  of  the  rocks  in  contact;  3dly,  mineral  characters; 
4thly,  included  fragments. 

Test  of  age  by  relative  position. — Unaltered  fossiliferous 
strata  of  every  age  are  met  with  reposing  immediately  on  plu- 
tonic  rocks,  as  at  Christiariia,  in  Norway,  where  the  Newer 
Pliocene  deposits  rest  on  granite ;  in  Auvergne,  where  the  fresh- 
water Eocene  strata,  and  at  Heidelberg,  on  the  Rhine,  where  the 
New  Red  sandstone  occupy  a  similar  place.  In  all  these,  and 
similar  instances,  inferiority  in  position  is  connected  with  the 
superior  antiquity  of  granite.  The  crystalline  rock  was  solid 
before  the  sedimentary  beds  were  superimposed,  and  the  latter 
usually  contain  in  them  rounded  pebbles  of  the  subjacent 
granite. 

Test  by  intrusion  and  alteration. — But  when  plutonic  rocks 
send  veins  into  strata,  and  alter  them  near  the  point  of  contact, 
in  the  manner  before  described  (p.  123.),  it  is  clear  that,  like  in- 
trusive traps,  they  are  newer  than  the  strata  which  they  invade 
and  alter.  Examples  of  the  application  of  this  test  will  be  given 
in  the  sequel. 

Test  by  mineral  composition. — Notwithstanding  a  general 
uniformity  in  the  aspect  of  plutonic  rocks,  we  have  seen  in  the 
9th  chapter  that  there  are  many  varieties,  such  as  Syenite,  Tal- 
cose  granite,  and  others.  One  of  these  varieties  is  sometimes 
found  exclusively  prevailing  throughout  an  extensive  region, 
where  it  preserves  a  homogeneous  character;  so  that  having 
ascertained  its  relative  age  in  one  place,  we  can  easily  recognize 
its  identity  in  others,  and  thus  determine  from  a  single  section 
the  chronological  relations  of  large  mountain  masses.  Having 
observed,  for  example,  that  the  syenitic  granite  of  Norway,  in 
which  the  mineral  called  zircon  abounds,  has  altered  the  Silurian 
strata  wherever  it  is  in  contact,  we  do  not  hesitate  to  refer  all 
masses  of  the  same  zircon-syenite  in  the  south  of  Norway  to  the 
same  era.  (See  p.  143.) 

Some  have  imagined  that  the  age  of  different  granites  might, 
to  a  great  extent,  be  determined  by  their  mineral  characters 
alone ;  syenite,  for  instance,  or  granite  with  hornblende,  being 
more  modern  than  common  or  micaceous  granite.  But  modern 
investigations  have  proved  these  generalizations  to  have  been 


PART  II.     CHAPTER  XXIV.  283 

Recent  and  Pliocene  Plutonic  Rocks. 

premature.  The  syenitic  granite  of  Norway  already  alluded  to 
may  be  of  the  same  age  as  the  Silurian  strata,  which  it  traverses 
and  alters,  or  may  belong  to  the  Old  Red  sandstone  period; 
whereas  the  granite  of  Dartmoor,  although  consisting  of  mica, 
quartz,  and  felspar,  is  newer  than  the  Coal.  (See  p.  499.) 

Test  by  included  fragments — This  criterion  can  rarely  be 
of  much  importance,  because  the  fragments  involved  in  granite 
are  usually  so  much  altered,  that  they  cannot  be  referred  with 
certainty  to  the  rocks  whence  they  were  derived.  In  the  White 
Mountains  in  North  America,  according  to  Professor  Hubbard,  a 
granite  vein  traversing  granite,  contains  fragments  of  slate  and 
trap,  which  must  have  fallen  into  the  fissure  when  the  fused  ma- 
terials of  the  vein  were  injected  from  below,*  and  thus  the  granite 
is  shown  to  be  newer  than  certain  superficial  and  slaty  trappean 
formations. 

Recent  and  Pliocene  plutonic  rocks,  why  invisible. — The 
explanation  already  given  in  the  8th  and  9th  chapters  of  the 
probable  relation  of  the  plutonic  to  the  volcanic  formations,  will 
naturally  lead  the  reader  to  infer  that  rocks  of  the  one  class  can 
never  be  produced  at  or  near  the  surface  without  some  members 
of  the  other  being  formed  below  simultaneously,  or  soon  after- 
wards. It  is  not  uncommon  for  lava  streams  to  require  more 
than  ten  years  to  cool  in  the  open  air ;  and  where  they  are  of 
great  depth,  a  much  longer  period.  The  melted  matter  poured 
from  Jorullo,  in  Mexico,  in  the  year  1759,  which  accumulated 
in  some  places  to  the  height  of  550  feet,  was  found  to  retain  a 
high  temperature  half  a  century  after  the  eruption.f  We  may 
conceive,  therefore,  that  great  masses  of  subterranean  lava  may 
remain  in  a  red-hot  or  incandescent  state  in  the  volcanic  foci  for 
immense  periods,  and  the  process  of  refrigeration  may  be  ex- 
tremely gradual.  Sometimes,  indeed,  this  process  may  be 
retarded  for  an  indefinite  period,  by  the  accession  of  fresh  sup- 
plies of  heat ;  for  we  find  that  the  lava  in  the  crater  of  Strom- 
boli,  one  of  the  Lipari  islands,  has  been  in  a  state  of  constant 
ebullition  for  the  last  two  thousand  years ;  and  we  must  suppose 
this  fluid  mass  to  communicate  with  some  caldron  or  reservoir 
of  fused  matter  below.  In  the  Isle  of  Bourbon,  also,  where  there 
has  been  an  emission  of  lava  once  in  every  two  years  for  a  long 
period,  the  lava  below  can  scarcely  fail  to  have  been  perma- 
nently in  a  state  of  liquefaction.  If  then  it  be  a  reasonable  con- 
jecture, that  about  2000  volcanic  eruptions  occur  in  the  course 
of  every  century,  either  above  the  waters  of  the  sea  or  beneath 

*  Silliraan's  Journ.  No.  69.  p.  123. 

t  See  Principles  of  Geology,  Index,  "Jorullo." 


284  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Relative  Age  and  Position  of  Volcanic  Rocks. 

them,*  it  will  follow,  that  the  quantity  of  plutonic  rock  generated, 
or  in  progress  during  the  recent  epoch,  must  already  have  been 
considerable. 

But  as  the  plutonic  rocks  originate  at  some  depth  in  the 
earth's  crust,  they  can  only  be  rendered  accessible  to  human 
observation  by  subsequent  upheaval  and  denudation.  Between 
the  period  when  a  plutonic  rock  crystallizes  in  the  subterranean 
regions,  and  the  era  of  its  protrusion  at  any  single  point  of  the 
surface,  one  or  two  geological  periods  must  usually  intervene. 
Hence,  we  must  not  expect  to  find  the  Recent  or  Newer  Pliocene 
granites  laid  open  to  view,  unless  we  are  prepared  to  assume  that 
sufficient  time  has  elapsed  since  the  commencement  of  the  Newer 
Pliocene  period  for  great  upheaval  and  denudation.  A  plutonic 
rock,  therefore,  must,  in  general,  be  of  considerable  antiquity  rela- 
tively to  the  fossiliferous  and  volcanic  formations,  before  it  be- 
comes extensively  visible.  As  we  know  that  the  upheaval  of  land 
has  been  sometimes  accompanied  in  South  America  by  volcanic 
eruptions  and  the  emission  of  lava,  we  may  conceive  the  more 
ancient  plutonic  rocks  to  be  forced  upwards  to  the  surface  by 
the  newer  rocks  of  the  same  class  formed  successively  below, 
— subterposition  in  the  plutonic,  like  superposition  in  the  sedi- 
mentary rocks,  being  usually  characteristic  of  a  newer  origin. 

In  the  accompanying  diagram,  Fig.  288.,  an  attempt  is  made 
to  show  the  inverted  order  in  which  sedimentary  and  plutonic 
formations  may  occur  in  the  earth's  crust. 

.  The  oldest  plutonic  rock,  No.  I.,  has  been  upheaved  at  suc- 
cessive periods  until  it  has  become  exposed  to  view  in  a  moun- 
tain chain.  This  protrusion  of  No.  I.  has  been  caused  by  the 
igneous  agency  which  produced  the  new  plutonic  rocks  Nos.  II. 
III.  and  IV.  Part  of  the  primary  fossiliferous  strata,  No.  1., 
has  also  been  raised  to  the  surface  by  the  same  gradual  pro- 
cess. It  will  be  observed  that  the  Recent  strata  No.  4.,  and  the 
Recent  granite  or  plutonic  rock  No.  IV.,  are  the  most  remote 
from  each  other  in  position,  although  of  contemporaneous  date. 
According  to  this  hypothesis,  the  convulsions  of  many  periods 
will  be  required  before  Recent  granite  will  be  upraised  so  as  to 
form  the  highest  ridges  and  central  axes  of  mountain-chains. 
During  that  time  the  Recent  strata  No.  4.  might  be  covered  by  a 
great  many  newer  sedimentary  formations. 

Tertiary  plutonic  rocks. — We  have  seen  that  great  upheaving 
movements  have  been  experienced  in  the  region  of  the  Andes, 
during  the  Recent  and  Newer  Pliocene  periods.  In  some  part, 
therefore,  of  this  chain,  if  any  where,  we  may  hope  to  discover 

*  See  Principles  of  Geology,  Index,  "  Volcanic  Eruptions.'' 


PART  II.     CHAPTER  XXIV. 


285 


Tertiary  Plutonic  Rocks  in  the  Andes. 


-     s 


2<* 

:•§ 


II 


tertiary  plutonic  rocks  laid  open  to  view.  What  we  already 
know  of  the  structure  of  the  Chilian  Andes  seems  to  realize  this 
expectation.  In  a  transverse  section,  examined  by  Mr.  Darwin, 
between  Valparaiso  and  Mendoza,  the  Cordillera  was  found  to 
consist  of  two  separate  and  parallel  chains,  formed  of  sedimen- 


286  LYELL'S  ELEMENTS  OF  GEOLOGY. 


Plutonic  Rocks  of  the  Chalk,  Oolite,  and  Lias. 


tary  rocks  of  different  ages,  the  strata  in  both  resting  on  plutonic 
rocks,  by  which  they  have  been  altered.  In  the  western  or  old- 
est range,  called  the  Peuquenes,  are  black  calcareous  clay-slates, 
rising  to  the  height  of  nearly  14,000  feet  above  the  sea,  in  which 
are  shells  of  the  genera  Gryphoea,  Turritella,  Terebratula,  and 
Ammonite.  These  rocks  are  supposed  to  be  of  the  age  of  the 
central  parts  of  the.  secondary  series  of  Europe.  They  are 
penetrated  and  altered  by  dikes  and  mountain  masses  of  a 
plutonic  rock,  which  has  the  texture  of  ordinary  granite,  but 
rarely  contains  quartz,  being  a  compound  of  albite  and  horn- 
blende. 

The  second  or  eastern  chain  consists  chiefly  of  sandstones 
and  conglomerates,  of  vast  thickness,  the  materials  of  which  are 
derived  from  the  ruins  of  the  western  chain.  The  pebbles  of 
the  conglomerates  are,  for  the  most  part,  rounded  fragments  of 
the  fossiliferous  slates  before  mentioned.  The  resemblance  of  the 
whole  series  to  certain  tertiary  deposits  on  the  shores  of  the 
Pacific,  not  only  in  mineral  character,  but  in  the  imbedded  lig- 
nite and  silicified  wood,  leads  to  the  conjecture  that  they  also  are 
tertiary.  Yet  these  strata  are  not  only  associated  with  trap 
rocks  and  volcanic  tuffs,  but  are  also  altered  by  a  granite  newer 
than  that  of  the  western  chain,  and  consisting  of  quartz,  felspar, 
and  talc.  They  are  traversed,  moreover,  by  dikes  of  the  same 
granite,  and  by  numerous  veins  of  iron,  copper,  arsenic,  silver, 
and  gold ;  all  of  which  can  be  traced  to  the  underlying  granite.* 
We  have,  therefore,  strong  ground  to  presume  that  the  plutonic 
rock,  here  exposed  on  a  large  scale  in  the  Chilian  Andes,  is  of 
later  date  than  certain  tertiary  formations. 

Cretaceous  period. — It  was  stated  (p.  145.)  that  chalk  as 
well  as  lias  have  been  altered  by  granite  in  the  eastern  Pyre- 
nees. Whether  such  granite  be  cretaceous  or  tertiary  cannot 
easily  be  decided. 

Suppose  b,  r,  d,   to   be  three 

Fig.  289.  members  of  the  Cretaceous  series, 

the  lowest. of  which,  ft,  has  been 
altered  by  the  granite  A,  the  modi- 
fying influence  not  having  extended 
"so  far  as  c,  or  having  but  slightly 
affected  its  lowest  beds.  Now  it 
can  rarely  be  possible  for  the  ge-. 
ologist  to  decide  whether  $e  beds 
d  existed  at  the  time  of  the  intrusion  of  A,  and  alteration  of  b 
and  c,  or  whether  they  were  subsequently  thrown  down  upon  c. 

*  Darwin,  pp.  390.  406. 


PART  II.     CHAPTER  XXIV. 


287 


Trap  Rocks  coeval  with  Oolite  and  Coal. 


As  some  Cretaceous  rocks,  however,  have  been  raised  to  the 
height  of  more  than  9000  feet  in  the  Pyrenees,  we  must  not 
assume  that  plutonic  formations  of  the  same  age  may  not  have 
been  brought  up  and  exposed  by  denudation,  at  the  heigut  of 
2000  or  3000  feet  on  the  flanks  of  that  chain. 

Period  of  Oolite  and  Lias. — In  the  department  of  the  Hautes 
Alpes,  in  France,  near  Vizille,  M.  Elie  de  Beaumont  traced  a 
black  argillaceous  limestone,  charged  with  belemnites,  to  within 

a  few  yards  of  a  mass 
Fig.  290.  of    granite.      Here    the 

limestone  begins  to  put 
on  a  granular  texture, 
but  is  extremely  fine- 
grained. When  nearer 
the  junction,  it  becomes 
grey,  and  has  a  saccha- 
roid  structure.  In  ano- 
ther locality,  near  Cham- 
poleon,  a  granite  com- 
posed of  quartz,  black 
mica,  and  rose-coloured 
felspar,  is  observed  partly 
to  overlie  the  secondary 
rocks,  producing  an  al- 
teration which  extends 
for  about  thirty  feet  down- 
wards, diminishing  in  the 
beds  which  lie  farthest 
from  the  granite.  (See  Fig.  290.)  In  the  altered  mass  the  argil- 
laceous beds  are  hardened,  the  limestone  is  saccharoid,  the  grits 
quartzose,  and  in  the  midst  of  them  is  a  thin  layer  of  an  imper- 
fect granite.  It  is  also  an  important  circumstance,  that  near  the 
point  of  contact,  both  the  granite  and  the  secondary  rocks 
become  metalliferous,  and  contain  nests  and  small  veins  of 
blende,  galena,  iron,  and  copper  pyrites.  The  stratified  rocks 
become  harder  and  more  crystalline,  but  the  granite,  on  the  con- 
trary, softer  and  less  perfectly  crystallized-near  the  junction.* 

Although  the  granite  is  incumbent  in  the  above  section,  (Fig. 
290.)  we  cannot  assume  that  it  overflowed  the  strata,  for  the  dis- 
turbances of  the  rocks  are  so  great  in  this  part  of  the  Alps  that 
they  seldom  retain  the  position  which  they  must  originally  have 
occupied. 


Junction  of  granite  with  Jurassic  or  Oolite  strata 
in  the  Jllps,  near  Champoleon. 


*  Elie  de  Beaumont,  sur  les  Montagnes  de  i'Oisans,  &c.,  Mem.  de  la  Soc. 
d'Hist.  Nat.  de  Paris,  tome  v. 


288      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Granite  of  Carboniferous  and  Old  Red  Sandstone  Period. 

A  considerable  mass  of  syenite,  in  the  Isle  of  Sky,  is  de- 
scribed by  Dr.  MacCulloch  as  intersecting  limestone  and  shale, 
which  are  of  the  age  of  the  lias.*  The  limestone,  which,  at  a 
greater  distance  from  the  granite,  contains  shells,  exhibits  no 
traces  of  them  near  its  junction,  where  it  has  been  converted 
into  a  pure  crystalline  marble. f 

At  Predazzo,  in  the  Tyrol,  secondary  strata,  some  of  which 
are  limestones  of  the  Oolitic  period,  have  been  traversed  and 
altered  by  plutonic  rocks,  one  portion  of  which  is  an  augitic  por- 
phyry, which  passes  insensibly  into  granite.  The  limestone  is 
changed  into  granular  marble,  with  a  band  of  serpentine  at  the 
junction.^: 

Carboniferous  period. — The  granite  of  Dartmoor,  in  Devon- 
shire, was  formerly  supposed  to  be  one  of  the  most  ancient  of 
the  plutonic  rocks,  but  is  now  ascertained  to  be  posterior  in  date 
to  the  culm-measures  of  that  county,  which,  from  their  position,, 
and  as  containing  true  coal-plants,  are  regarded  by  Professor 
Sedgwick  and  Mr.  Murchison  as  members  of  the  true  carbonife- 
rous series.  This  granite,  like  the  syenitic  granite  of  Christi- 
ania,  has  broken  through  the  stratified  formations  without  much 
changing  their  strike.  Hence,  on  the  north-west  side  of  Dart- 
moor, the  successive  members  of  the  culm-measures  abut  against 
the  granite,  and  become  metamorphic  as  they  approach.  These 
strata  are  also  penetrated  by  granite  veins  and  plutonic  dikes, 
called  "  elvans."§  The  granite  of  Cornwall  is  probably  of  the 
same  date,  and,  therefore,  as  modern  as  the  Carboniferous  strata, 
if  not  much  newer. 

Old  Red  sandstone  period. — The  plutonic  rocks  of  the  Mal- 
vern  hills,  in  Worcestershire,  consist  of  a  granitic  compound  of 
quartz,  felspar,  and  hornblende,  or  occasionally  of  quartz,  mica, 
and  felspar,  which  passes  into  syenite  and  greenstone.||  This 
rock  has  altered  the  adjacent  Silurian  strata  into  well  character- 
ized metamorphic  schists,  principally  chloride  and  micaceous- 
schist,  with  some  gneiss,  and  has  dislocated  and  reversed  the 
position  of  the  beds  of  the  Silurian  and  Old  Red  sandstone. 
There  are  indications,  says  Mr.  Murchison,  of  several  periods 
of  movement,  by  which  the  strata  were  forced  up  and  folded 
back,  but  the  chief  outburst  was  after  the  accumulation  of  the 
Silurian  and  part  of  the  Old  Red  system,  and  anterior  to  the  for- 
mation of  the  coal-beds,  which  are  undisturbed.lF 


*  See  Murchison,  Geol.  Trans.,  2d  series,  vol.  ii.  part  ii.  pp.  311 — 321. 

t  Western  Islands,  vol.  I  p.  330.  plate  18.  figs.  3,  4. 

t  Von  Buch,  Annales  de  Chimie,  &c. 

§  Proceedings  of  Geol.  Soc.,  vol.  ii.  p.  562. 

II  Mr.  L.  Horner,  Geol.  Trans.,  1st  ser.,  vol.  i.  p.  281. 

tf  Silurian  System,  p  425. 


PART  II.     CHAPTER  XXIV.  289 


Granite  of  Silurian  Period. 


Silurian  period. — I  have  already  alluded  to  the  granite  near 
Christiania,  in  Norway,  as  being  newer  than  the  Silurian  strata 
of  that  region.  Its  posteriority  in  date  to  limestones  containing 
orthocerata  and  trilobites,  has  long  been  celebrated,  it  being 
twenty-five  years  since  Von  Buch  first  announced  the  discovery. 
The  proofs  consist  in  the  penetration  of  granite  veins  into  the 
shale  and  limestone,  and  the  alteration  of  the  strata,  for  a  con- 
siderable distance  from  the  point  of  contact,  both  of  these  veins 
and  the  central  mass  from  which  they  emanate.  (See  p.  129.) 
Von  Buch  supposed  that  the  plutonic  rock  alternated  with  the 
fossiliferous  strata,  and  that  large  masses  of  granite  were  some- 
times incumbent  upon  the  strata  ;  but  this  idea  was  erroneous, 
and  arose  from  the  fact  that  the  beds  of  shale  and  limestone 
often  dip  towards  the  granite  up  to  the  point  of  contact,  appear- 
ing as  if  they  would  pass  under  it  in  mass,  as  at  a,  Fig.  291., 

Fig.  291. 


Silurian.  Granite.  Silurian  strata. 

and  then  again  on  the  opposite  side  of  the  same  mountain,  as  at 
&,  dip  away  from  the  same  granite.  When  the  junctions,  how- 
ever, are  carefully  examined,  it  is  found  that  the  plutonic  rock 
intrudes  itself  in  veins,  and  nowhere  covers  the  fossiliferous 
strata  in  large  overlying  masses,  as  is  so  commonly  the  case 
with  trappean  formations.* 

Now  this  granite,  which  is  more  modern  than  the  Silurian 
strata  of  Norway,  also  sends  veins  in  the  same  country  into  an 
ancient  formation  of  gneiss ;  and  the  relations  of  the  plutonic 
rock  and  the  gneiss,  at  their  junction,  are  full  of  interest  when 
we  duly  consider  the  wide  difference  of  epoch  which  must  have 
separated  their  origin. 

The  length  of  this  interval  of  time  is  attested  by  the  following 
facts : — The  fossiliferous,  or  transition  beds,  rest  unconformably 
upon  the  truncated  edges  of  the  gneiss,  the  inclined  strata  of 
which  had  been  disturbed  and  denuded  before  the  sedimentary 
beds  were  superimposed.  (See  Fig.  292.) 

The  signs  of  denudation  are  twofold ;  first,  the  surface  of  the 
gneiss  is  seen  occasionally  on  the  removal  of  the  newer  beds, 
containing  organic  remains,  to  be  scored  and  polished ;  secondly, 

*  See  the  Gsea  Norvegica  and  other  works  of  Keilhau,  with  whom  I  examined 
this  country. 


290 


LYELL'S  ELEMENTS  OF  GEOLOGY. 


Granite  altering  Silurian  Strata Most  ancient  Granites. 


Fig.  292. 


Silurian  strata. 


Gneiss.  Granite.  Gneiss. 

Granite  sending  veins  into  Silurian  strata  and  Gneiss,—  Christiania,  Norway. 

pebbles  of  gneiss  have  been  found  in  some  of  the  transition 
strata.  Between  the  origin,  therefore,  of  the  gneiss  and  the  gra- 
nite there  intervened,  first,  the  period  when  the  strata  of  gneiss 
were  inclined ;  secondly,  the  period  when  they  were  denuded ; 
thirdly,  the  period  of  the  deposition  of  .the  transition  deposits. 
Yet  the  granite  produced,  after  this  long  interval,  is  often  so  inti- 
mately blended  with  the  ancient  gneiss,  at  the  point  of  junction, 
that  it  is  impossible  to  draw  any  other  than  an  arbitrary  line  of 
separation  between  them ;  and  where  this  is  not  the  case,  tortu- 
ous veins  of  granite  pass  freely  through  gneiss,  ending  some- 
times in  threads,  as  if  the  older  rock  had  offered  no  resistance 
to  their  passage.  It  seems  necessary,  therefore,  to  conceive  that 
the  gneiss  was  softened  and  more  or  less  melted  when  penetrated 
by  the  granite.  But  had  such  junctions  alone  been  visible,  and 
had  we  not  learnt,  from  other  sections,  how  long  a  period  elapsed 
between  the  consolidation  of  the  gneiss  and  the  injection  of  this 
granite,  we  might  have  suspected  that  the  gneiss  was  scarcely 
solidified,  or  had  not  yet  assumed  its  complete  metamorphic  cha- 
racter, when  invaded  by  the  plutonic  rock.  From  this  example 
we  may  learn  how  impossible  it  is  to  conjecture  whether  certain 
granites  in  Scotland,  and  other  countries,  which  send  veins  into 
gneiss  and  other  metamorphic  rocks,  are  primary,  or  whether 
they  may  not  belong  to  some  secondary  or  tertiary  period. 

Most  ancient  granites. — It  is  riot  half  a  century  since  the 
doctrine  was  very  general  that  all  granitic  rocks  were  primitive, 
that  is  to  say,  that  they  originated  before  the  deposition  of  the 
first  sedimentary  strata,  and  before  the  creation  of  organic 
beings.  (See  p.  23.)  But  so  greatly  are  our  views  now  changed, 
that  we  find  it  no  easy  task  to  point  out  a  single  mass  of  granite 
demonstrably  more  ancient  than  all  the  known  fossiliferous  depo- 
sits. Could  we  discover  some  Lower  Cambrian  strata  resting 
immediately  on  granite,  there  being  no  alterations  at  the  point 
of  contact,  nor  any  intersecting  granitic  veins,  we  might  then 
affirm  the  plutonic  rock  to  have  originated  before  the  oldest 
known  fossiliferous  strata.  Still  it  would  be  presumptuous  to 


PART  II.     CHAPTER  XXIV.  291 

Protrusion  of  Solid  Granite. 

suppose  that  when  a  small  part  only  of  the  globe  has  been  inves- 
tigated, we  are  acquainted  with  the  oldest  fbssiliferous  strata  in 
the  crust  of  our  planet.  Even  when  these  are  found,  we  cannot 
assume  that  there  never  were  any  antecedent  strata  containing 
organic  remains,  which  may  have  become  metamorphic.  If  we 
find  pebbles  of  granite  in  a  conglomerate  of  the  Lower  Cam- 
brian system,  we  may  then  feel  assured  that  the  parent  granite 
was  formed  before  the  Lower  Cambrian  formation.  But  if  the 
incumbent  strata  be  merely  Silurian  or  Upper  Cambrian,  the 
fundamental  granite,  although  of  high  antiquity,  may  be  poste- 
rior in  date  to  known  fossiliferous  formations. 

Protrusion  of  solid  granite. — In  part  of  Sutherlandshire, 
near  Brora,  common  granite,  composed  of  felspar,  quartz,  and 
mica,  is  in  immediate  contact  with  Oolitic  strata,  and  has  clearly 
been  elevated  to  the  surface  at  a  period  subsequent  to  the  deposi- 
tion of  those  strata.*  Professor  Sedgwick  and  Mr.  Murchison 
conceive  that  this  granite  has  been  upheaved  in  a  solid  form  ; 
and  that  in  breaking  through  the  submarine  deposits,  with  which 
it  was  not  perhaps  originally  in  contact,  it  has  fractured  them  so 
as  to  form  a  breccia  along  the  line  of  junction.  This  breccia 
consists  of  fragments  of  shale,  sandstone,  and  limestone,  with 
fossils  of  the  oolite,  all  united  together  by  .a  calcareous  cement. 
The  secondary  strata,  at  some  distance  from  the  granite,  are  but 
slightly  disturbed,  but  in  proportion  to  their  proximity  the 
amount  of  dislocation  becomes  greater. 

If  we  admit  that  solid  hypogene  rocks,  whether  stratified  or 
unstratified,  have  in  such  cases  been  driven  upwards,  so  as  to 
pierce  through  yielding  sedimentary  deposits,  we  shall  be  enabled 
to  account  for  many  geological  appearances  otherwise  inexplica- 
ble. Thus,  for  example,  at  Weinbohla  and  Hohnstein,  near 
Meissen,  in  Saxony,  a  mass  of  granite  has  been  observed  cover- 
ing strata  of  the  cretaceous  and  oolitic  periods  for  the  space  of 
between  300  and  400  yards  square.  It  appears  clearly  from  a 
recent  memoir  of  Dr.  B.  Cotta  on  this  subject,!  that  the  granite 
was  thrust  into  its  actual  position  when  solid.  There  are  no 
intersecting  veins  at  the  junction — no  alteration  as  if  by  heat, 
but  evident  signs  of  rubbing,  and  a  breccia  in  some  places,  in 
which  pieces  of  granite  are  mingled  with  broken  fragments  of 
the  secondary  rocks.  As  the  granite  overhangs  both  the  lias 
and  chalk,  so  the  lias  is  in  some  places  bent  over  strata  of  the 
cretaceous  era. 

Age  of  the  granite  of  Arran — In  this  island,  the  largest  in 

*  Murchison,  Geol.  Trans.  3d  series,  vol.  ii.  p.  307. 
t  Geognostische  Wanderungen,  Leipzic,  1838. 


292  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Age  of  the  Granite  of  the  Isle  of  Arran. 

the  Firth  of  Clyde,  on  the  west  coast  of  Scotland,  the  four  great 
classes  of  rocks,  the  fossiliferous,  volcanic,  plutonic  and  meta- 
morphic,  are  all  conspicuously  displayed  within  a  very  small 
area,  arid  with  their  peculiar  characters  strongly  contrasted.  In 
the  north  of  the  island  the  granite  rises  to  the  height  of  nearly 
3000  feet  above  the  sea,  terminating  in  mountainous  peaks.  On 
the  flanks  of  the  same  mountains  are  chloritic-schists,  blue  roof- 
ing-slate, and  other  rocks  of  the  metamorphic  order  (A),  into 

Fig.  293.  Section  of  Arran. 

B 


A,  Crystalline,  or  metamorphic  schist.  B,  Granite, 

c,  Conglomerate,  sandstone,  limestone,  and  shale.        D,  Trap. 

which  the  granite  (B)  sends  veins.  These  schists  are  highly 
inclined.  On  their  truncated  edges  rest  unconformable  beds  of 
conglomerate  and  sandstone,  to  which  succeed  various  shales 
and  limestones,  containing  fossils  of  the  carboniferous  period. 
All  these  secondary  strata  (c)  are  much  tilted  and  inclined  near 
the  hypogene  rocks ;  but  are  horizontal  at  a  distance  from  them, 
as  in  the  south  of  Arran.  Lastly,  the  volcanic  rocks  (D),  con- 
sisting of  greenstone,  pitchstone,  claystone,  porphyry,  and  other 
varieties,  traverse  all  the  preceding  formations,  cutting  through 
the  granite  in  dikes  (d),  as  well  as  through  the  sandstone ; 
which  last  they  also  overlie  in  dense  masses,  from  50  to  700 
feet  in  thickness. 

Now  as  the  different  kinds  of  trap  intersect  all  the  other  form- 
ations, they  are  certainly  the  newest  rocks  in  Arran.  The  red 
sandstone  and  other  secondary  strata  are  older  than  the  trap, 
but  newer  than  the  metamorphic  schists,  for  the  red  sandstone 
conglomerates  not  only  rest  unconformably  upon  the  schists,  but 
contain  rounded  pebbles  of  those  crystalline  strata.  It  is  equally 
certain  that  the  schists  are  the  oldest  rocks  in  the  island  :  they 
are  more  ancient  than  the  trap  and  red  sandstone,  for  reasons 
already  stated  ;  and  the  granite  must  be  of  newer  origin,  because 
it  penetrates  them  in  veins.  The  only  chronological  point, 
therefore,  in  which  there  can  be  any  ambiguity,  relates  to  the 
plutonic  formations.  They  are  more  modern,  as  before  remarked, 


PART  II.     CHAPTER  XXIV.  293 


Age  of  the  Granite  of  the  Isle  of  Arran. 


than  the  crystalline  schists  ;  but  can  we  decide  them  to  be  like- 
wise younger  than  the  secondary  sandstones  ? 

Now  it  is  a  curious  and  most  striking  fact,  that  no  pebbles  of 
granite  have  ever  been  found  in  the  conglomerates  of  the  red 
sandstone  in  Arran,  although  careful  search  has  been  made  for 
them  by  many  geologists ;  and  although  puddingstones  in  gene- 
ral are  chiefly  made  up  of  fragments  of  older  rocks  of  the  imme- 
diate vicinity.  The  total  absence  of  such  pebbles  has  justly  been 
a  theme  of  wonder  to  those  who  have  visited  Arran,  and  have 
seen  that  the  conglomerates  are  several  hundred  feet  in  thick- 
ness, and  that  they  occur  at  the  base  of  the  granite  mountains, 
which  tower  above  them  in  far  bolder  and  more  picturesque 
forms  than  those  of  similar  composition  in  other  parts  of  Scot- 
land. We  may  at  once  infer,  with  confidence,  that  when  the 
sandstone  and  conglomerate  were  formed,  no  granite  had  reached 
the  surface,  or  had  been  exposed  to  denudation  in  this  region : 
the  crystalline  schists  were  ground  into  sand  and  shingle  when 
these  puddingstones  were  accumulated,  but  the  waves  had  never 
acted  upon  the  granite  which  sends  its  veins  into  the  schist. 
Are  we  then  to  conclude,  that  the  schists  suffered  denudation 
before  they  had  been  invaded  by  granite?  This  opinion,  al- 
though it  cannot  be  disproved,  is  by  no  means  fully  borne  out. 
by  the  evidence.  At  the  time  when  the  red  sandstone  was 
formed,  the  metamorphic  strata  may  have  formed  islands  in  the 
sea,  as  in  Fig.  294.,  over  which  the  breakers  rolled,  or  from 

Fig.  294. 


which  torrents  and  rivers  descended,  carrying  down  gravel  and 
sand.  The  plutonic  rock  (B)  may  have  been  previously  injected 
at  a  certain  depth  below,  and  yet  may  never  have  been  exposed 
to  denudation. 

As  to  the  time  and  manner  of  the  subsequent  protrusion  of 
the  hypogene  rocks  in  Arran,  these  are  questions  into  which  I 
have  not  space  to  enter  at  present :  I  shall  merely  observe,  that 
those  crystalline  rocks  may  have  been  thrust  up  bodily,  in  a 
solid  form ;  and  it  is  clear  that,  during  or  since  the  period  of 
their  emergence,  they  have  undergone  great  aqueous  denuda- 
tion. This  action  is  confirmed  by  three  distinct  kinds  of  proofs : 
1st,  The  occurrence  of  scattered  pebbles  and  huge  erratic  blocks 
of  granite  arid  schist  over  the  surface  of  Arran  and  the  adjacent^ 
z* 


294  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Different  Ages  of  Metamorphic  Rocks. 

mainland ;  2dly,  The  abrupt  truncation  of  dikes,  such  as  those 
at  d  (Fig.  293.),  cut  off  on  the  surface  of  the  granite ;  3dly,  The 
fact  -that  not  only  the  secondary  strata  but  the  enormous  masses 
of  trap  which  accompany  and  overlie  them,  terminate  suddenly 
on  reaching  the  borders  of  the  granite  and  schist,  towards  which 
they  often  present  a  steep  escarpment,  and  over  which,  for 
some  distance  at  least,  they  must  originally  have  extended.* 


CHAPTER  XXV. 


ON  THE   DIFFERENT   AGES   OF   THE   METAMORPHIC   ROCKS. 

Age  of  each  set  of  metamorphic  strata  twofold — Test  of  age  by  fossils  and 
mineral  character  not  available — Test  by  superposition  ambiguous — Conversion 
of  dense  masses  of  fossiliferous  strata  into  metamorphic  rocks — Limestone  and 
shale  of  Carrara — Metamorphic  strata  of  modern  periods  in  the  Alps  of  Switzer- 
land and  Savoy — Why  the  visible  crystalline  strata  are  none  of  them  very 
modern — Order  of  succession  in  metamorphic  rocks — Uniformity  of  mineral  cha- 
racter— Why  the  metamorphic  strata  are  less  calcareous  than  the  fossiliferous. 

ACCORDING  to  the  theory  adopted  in  the  llth  chapter,  the 
age  of  each  set  of  metamorphic  strata  is  twofold,  they  have  been 
deposited  at  one  period,  they  have  become  crystalline  at  another. 
We  can  rarely  hope  to  define  with  exactness  the  date  of  both 
these  periods,  the  fossils  having  been  destroyed  by  plutonic 
action,  and  the  mineral  characters  being  the  same,  whatever  the 
age.  Superposition  itself  is  an  ambiguous  test,  especially  when 
we  desire  to  determine  the  period  of  crystallization.  Suppose, 
for  example,  we  are  convinced  that  certain  metamorphic  strata 
in  the  Alps,  which  are  covered  by  cretaceous  beds,  are  altered 
lias ;  this  lias  may  have  assumed  its  crystalline  texture  in  the 
cretaceous  or  in  some  tertiary  period,  the  Eocene  for  example. 
If  in  the  latter,  it  should  be  called  Eocene,  when  regardecj  as  a 
metamorphic  rock,  although  it  be  liassic,  when  considered  in 
reference  to  the  era  of  its  deposition.  According  to  this  view, 
the  superposition  of  chalk  does  not  prevent  the  subjacent  meta- 
morphic rock  from  being  Eocene.  If,  however,  in  the  progress 

*  In  the  works  of  Drs.  Hutton  and  MacCulloch,  and  in  the  Memoirs  of  Messrs. 
Von  Dechen  and  Oeynhausen,  and  that  of  Professor  Sedgwick  and  Mr.  Murchi- 
eon,  (Geol.  Trans.  2d  series)  and  others,  whose  observations  I  have  verified  on 
the  spot,  the  reader  will  find  a  full  description  of  the  geology  of  Arran. 


PART  II.    CHAPTER  XXV.  295 

Age  of  the  Metamorphic  Rocks  of  the  Northern  Apennines. 

of  science,  we  should  succeed  in  ascertaining  the  twofold  chro- 
nological relations  of  the  metamorphic  formations,  it  might  be 
useful  to  adopt  a  twofold  terminology.  We  might  call  the  strata 
above  alluded  to  Liassic-Eocene,  or  Liassic-Cretaceous  ;  the  first 
term  referring  to  the  era.  of  deposition,  the  second  to  that-  of 
crystallization.  According  to  this  method,  the  chlorite-schist, 
mica-schist,  and  gneiss  of  the  Malvern  Hills,  would  belong  to 
the  Silurian  Old  Red  sandstone  period,  because  they  are  Silurian 
strata  altered  into  metamorphic  rocks  during  the  deposition  of 
the  Old  Red  sandstone.  (See  p.  288.) 

We  have  seen,  when  discussing  the  ages  of  the  plutonic  rocks, 
that  examples  occur  of  various  primary,  secondary,  and  tertiary 
deposits  converted  into  metamorphic  strata,  near  their  contact 
with  granite.  There  can  be  no  doubt  in  these  cases  that  strata, 
once  composed  of  mud,  sand,  and  gravel,  or  of  clay,  marl,  and 
shelly  limestone,  have  for  the  distance  of  several  yards,  and  in 
some  instances  several  hundred  feet,  been  turned  into  gneiss, 
mica-schist,  hornblende-schist,  chlorite-schist,  quartz  rock,  sta- 
tuary marble,  and  the  rest.  (See  Chapters  10.  and  11.) 

But  when  the  metamorphic  action  has  operated  on  a  grander 
scale,  it  tends  entirely  to  destroy  all  monuments  of  the  date  of 
its  development.  It  may  be  easy  to  prove  the  identity  of  two 
different  parts  of  the  same  stratum ;  one,  where  the  rock  has 
been  in  contact  with  a  volcanic  or  plutonic  mass,  and  has  been 
changed  into  marble  or  hornblende-schist,  and  another  not  far 
distant,  where  the  same  bed  remains  unaltered  and  fossiliferous ; 
but  when  we  have  to  compare  two  portions  of  a  mountain  chain — 
the  one  metamorphic,  and  the  other  unaltered — all  the  labour  and 
skill  of  the  most  practised  observers  are  required.  I  shall  men- 
tion one  or  two  examples  of  alteration  on  a  grand  scale,  in  order 
to  explain  to  the  student  the  kind  of  reasoning  by  which  we  are 
led  to  infer  that  dense  masses  of  fossiliferous  strata  have  been 
converted  into  crystalline  rocks. 

Northern  Appenines. — Carrara. — The  celebrated  marble  of 
Carrara,  used  in  sculpture,  was  once  regarded  as  a  type  of  primi- 
tive limestone.  It  abounds  in  the  mountains  of  Massa  Carrara, 
or  the  "Apuan  Alps,"  as  they  have  been  called,  the  highest 
peaks  of  which  are  nearly  6000  feet  high.  Its  great  antiquity 
was  inferred  from  its  mineral  texture,  from  the  absence  of  fossils, 
and  its  passage  downwards  into  talc-schist  and  garnetiferous 
mica-schist  ;  "these  rocks  again  graduating  downwards  into 
gneiss,  which  is  penetrated,  at  Forno,  by  granite  veins.  Now 
the  researches,  of  MM.  Savi,  Boue,  Pareto,  Guidoni,  De  la 
Beche,  and  especially  Hoffman,  have  demonstrated  that  this 
marble,  once  supposed  to  be  formed  before  the  existence  of 


296      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Age  of  Met^nrorphic  Rocks  of  the  Swiss  Alps. 

organic  beings,4*^,  in  fact,  an  altered  limestone  of  the  oolitic 
period,  and  the  underlying  crystalline  schists  are  secondary 
sandstones  and  shales,  modified  by  plutonic  action.  In  order  to 
establish  these  conclusions  it  was  first  pointed  out,  that  the  cal- 
careous rocks  bordering  the  Gulf  of  Spezia,  and  abounding  in 
oolitic  fossils,  assume  a  texture  like  that  of  Carrara  marble,  in 
proportion  as  they  are  more  and  more  invaded  by  certain  trap- 
pean  and  plutonic  rocks,  such  as  diorite,  euphodite,  serpentine, 
and  granite,  occurring  in  the  same  country. 

It  was  then  observed  that,  in  places  where  the  secondary  form- 
ations are  unaltered,  the  uppermost  consist  of  common  Apen- 
nine  limestone  with  nodules  of  flint,  below  which  are  shales,  and 
at  the  base  of  all,  argillaceous  and  siliceous  sandstones.  In  the 
limestone,  fossils  are  frequent,  but  very  rare  in  the  underlying 
shale  and  sandstone.  Now  a  gradation  has  been  traced  laterally 
from  these  rocks  into  another  and  corresponding  series,  which  is 
completely  metamorphic ;  for  at  the  top  of  this  we  find  a  white 
granular  marble,  wholly  .devoid  of  fossils,  and  almost  without 
stratification,  in  which  there  are  no  nodules  of  flint,  but  in  its 
place  siliceous  matter  disseminated  through  the  mass  in  the  form 
of  prisms  of  quartz.  Below  this,  and  in  place  of  the  shales,  are 
talc-schists,  jasper,  and  hornstone ;  and  at  the  bottom,  instead 
of  the  siliceous  and  argillaceous  sandstones,  are  quartzite  and 
gneiss.*  Had  these  secondary  strata  of  the  Apennines  under- 
gone universally  as  great  an  amount  of  transmutation,  it  would 
have  been  impossible  to  form  a  conjecture  respecting  their  true 
age ;  and  then,  according  to  the  common  method  of  geological 
classification,  they  would  have  ranked  as  primary  rocks.  In  that 
case  the  date  of  their  origin  would  have  been  thrown  back  to  an 
era  antecedent  to  the  deposition  of  the  Lower  Cambrian  strata, 
although  in  reality  they  were  formed  in  the  oolitic  period,  and 
altered  at  some  subsequent  and  unknown  epoch. 

Alps  of  Switzerland. — In  the  Alps,  analogous  conclusions 
have  been  drawn  respecting  the  alteration  of  strata  on  a  still 
more  extended  scale.  In  the  eastern  part  of  that  chain,  some 
of  the  primary  fossiliferous  strata,  as  well  as  the  older  secondary 
formations,  together  with  the  oolitic  and  cretaceous  rocks,  are 
distinctly  recognizable.  Tertiary  deposits  also  appear  in  a  less 
elevated  position  on  the  flanks  of  the  Eastern  Alps ;  but  in  the 
Central  or  Swiss  Alps,  the  primary  fossiliferous,  and  older  second- 
ary formations  disappear,  and  the  cretaceous,  oolitic,  and  liassic 
strata  graduate  insensibly  into  metamorphic  rocks,  consisting  of 

*  See  notices  of  Savi,  Hoffman,  and  others,  referred  to  by  Boue,  Bull,  de  la  Soe. 
Geol.  de  France,  torn.  v.  p.  317.  and  torn.  iii.  p.  44. 


PART  II.     C1IA 


Age  of  Metamorphic  Rocks 


granular  limestone,  talc-schist,  talcose-gneiss,rmca^^usschist," 
and  other  varieties.  In  regard  to  the  age  of  this  vast  assemblage 
of  crystalline  strata,  we  can  merely  affirm  that  some  of  the  upper 
portions  are  altered  newer  secondary  deposits :  but  we  cannot 
avoid  suspecting  that  the  disappearance  both  of  the  older  second- 
ary and  primary  fossiliferous  rocks  may  be  owing  to  their  hav- 
ing been  all  converted  in  this  region  into  crystalline  schist. 

It  is  difficult  to  convey  to  those  who  have  never  visited  the 
Alps  a  just  idea  of  the  various  proofs  which  concur  to  produce 
this  conviction.  In  the  first  place,  there  are  certain  points  where 
strata  of  the  Oolite,  Lias,  and  Chalk  have  been  turned  into  gra- 
nular marble,  gneiss,  and  other  metamorphic  schists,  near  their 
contact  with  granite.  This  fact  shows  undeniably  that  plutonic 
causes  continued  to  be  in  operation  in  the  Alps  down  to  a  late 
period,  even  after  the  deposition  of  some  of  the  newer  secondary 
formations.  Having  established  this  point,  we  are  the  more 
willing  to  believe  that  many  inferior  fossiliferous  rocks,  probably 
exposed  for  longer  periods  to  a  similar  action,  may  have  become 
metamorphic  to  a  still  greater  extent. 

We  also  discover  in  parts  of  the  Swiss  Alps  dense  masses  of 
strata  of  the  age  of  the  Green-sand  and  Chalk,  which  have  as- 
sumed that  semi-crystalline  texture  which  Werner  called  transi- 
tion, and  which  naturally  led  his  followers,  who  attached  great 
importance  to  mineral  characters  taken  alone,  to  class  them  as 
transition  formations,  or  as  groups  older  than  the  lowest  second- 
ary rocks.  (See  p.  154.)  Now,  it  is  probable  that  these  strata 
have  been  affected,  although  in  a  less  intense  degree,  by  that 
same  plutonic  action  which  has  entirely  altered  and  rendered 
metamorphic  so  many  of  the  subjacent  formations ;  for  in  the 
Alps,  this  action  has  by  no  means  been  confined  to  the  immediate 
vicinity  of  granite.  Granite,  indeed,  and  other  plutonic  rocks 
rarely  make  their  appearance  at  the  surface,  notwithstanding  the 
deep  ravines  which  lay  open  to  view  the  internal  structure  of 
these  mountains.  That  they  exist  below  at  no  great  depth  we 
cannot  doubt,  and  we  have  already  seen  (p.  127.)  that  at  some 
points,  as  in  the  Valorsine,  near  Mont  Blanc,  granite  and  grani- 
tic veins  are  observable,  piercing  through  talcose  gneiss,  which 
passes  insensibly  upwards  into  secondary  strata. 

It  is  certainly  in  the  Alps  of  Switzerland  and  Savoy,  more 
than  in  any  other  district  in  Europe,  that  the  geologist  is  prepared 
to  meet  with  the  signs  of  an  intense  development  of  plutonic 
action ;  for  here  we  find  the  most  stupendous  monuments  of 
mechanical  violence,  by  which  strata  thousands  of  feet  thick  have 
been  bent,  folded,  and  overturned.  (See  p.  74.)  It  is  here 
that  marine  secondary  formations  of  a  comparatively  modern 


298      LYELL'S  ELEMENTS  OF  GEOLOGY. 

Age  of  Mejajfcorphic  Rocks  of  the  Swiss  Alps. 

date,  such  as  tff<f  oolitic  and  cretaceous,  have  been  upheaved  to 
the  height  of  10,000,  or  even  12,000  feet  above  the  level  of  the 
sea ;  and  even  tertiary  strata,  apparently  of  the  Miocene  era, 
have  been  raised  to  an  elevation  of  4000  or  5000  feet,  so  as  to 
rival  in  height  the  loftiest  mountains  in  Great  Britain. 

If  the  reader  will  consult  the  works  of  many  eminent  geolo- 
gists who  have  explored  the  Alps,  especially  those  of  MM.  De 
Beaumont,  Studer,  Necker,  and  Boue,  he  will  learn  that  they  all 
share,  more  or  less  fully,  in  the  opinions  above  expressed.  It 
has,  indeed,  been  stated  by  MM.  Studer  and  Hugi,  that  there  are 
Complete  alternations  on  a  large  scale  of  secondary  strata,  con- 
taining fossils,  with  gneiss  and  other  rocks,  of  a  perfectly  meta- 
morphic  structure.  I  have  visited  some  of  the  most  remarkable 
localities  referred  to  by  these  authors,  but  although  agreeing  with 
them  that  there  are  passages  from  the  fossiliferous  to  the  meta- 
morphic  series  far  from  the  contact  of  granite  or  other  plutonic 
rocks,  I  was  unable  to  convince  myself  that  the  distinct  al- 
ternations of  highly  crystalline,  with  unaltered  strata  above 
alluded  to,  might  not  admit  of  a  different  explanation.  In  one 
of  the  sections  described  by  M.  Studer  in  the  highest  of  the  Ber- 
nese Alps,  namely  in  the  Roththal,  a  valley  bordering  the  line 
of  perpetual  snow  on  the  northern  side  of  the  Jungfrau,  I  observ- 
ed a  mass  of  gneiss  1000  feet  thick,  and  15,000  feet  long,  not 
only  resting  upon,  but  also  again  covered  by  strata  containing 
oolitic  fossils.  These  anomalous  appearances  may  partly  be 
explained  by  supposing  great  solid  wedges  of  intrusive  gneiss  to 
have  been  forced  in  laterally  between  strata  to  which  I  found 
them  to  be  in  many  sections  unconformable.  The  superposition, 
also,  of  the  gneiss  to  the  oolite  may,  in  some  cases,  be  due  to  a 
reversal  of  the  original  position  of  the  beds  in  a  region  where 
the  convulsions  have  been  on  so  stupendous  a  scale. 

On  the  Sattel  also,  at  the  base  of  the  Gestellihorn,  above 
Enzen,  in  the  valley  of  Urbach,  near  Meyringen,  some  of  the 
intercalations  of  gneiss  between  fossiliferous  strata  may,  I  con- 
ceive, be  ascribed  to  mechanical  derangement.  Almost  any 
hypothesis  of  repeated  changes  of  position  may  be  resorted  to 
in  a  region  of  such  extraordinary  confusion.  The  secondary 
strata  may  first  have  been  vertical,  and  then  certain  portions 
may  have  become  metamorphic  (the  plutonic  influence  ascending 
from  below)  while  intervening  strata  remained  unchanged.  The 
whole  series  of  beds  may  then  agani  have  been  thrown  into  a 
nearly  horizontal  position,  giving  rise  to  the  superposition  of 
crystalline  upon  fossiliferous  formations. 

It  was  remarked,  in  the  last  chapter,  that  as  the  hypogene 
rocks,  both  stratified  and  unstratified,  crystallize  originally  at  a 


PART  II.     CHAPTER  XXV.  299 

Age  of  Metamorphic  Rocks,  and  Order  of  Superposition. 

certain  depth  beneath  the  surface,  they  must  always,  before  they 
are  upraised  and  exposed  at  the  surface,  be  of  considerable  anti- 
quity, relatively  to  a  large  portion  of  the  fossiliferous  and  vol- 
canic rocks.  They  may  be  forming  at  all  periods ;  but  before 
any  of  them  can  become  visible,  they  must  be  raised  above  the 
level  of  the  sea,  and  some  of  the  rocks  which  previously  con- 
cealed them  must  have  been  removed  by  denudation.  If  the 
student  will  refer  to  the  frontispiece,  he  will  see  that  the  strata 
A,  which  were  the  last  deposited,  are  every  where  hidden  from 
human  observation  by  the  sea,  while  the  contemporaneous  meta- 
morphic  rocks  C  are  concealed  at  a  still  greater  depth,  as  are 
also  the  plutonic  rocks  D  of  the  same  age.  He  will  also  observe 
that  the  strata  C,  which  have  recently  become  metamorphic,  are 
not  parts  of  A,  nor  even  of  the  groups  immediately  antecedent 
in  date  a,  6,  c,  but  they  are  portions  of  much  older  formations, 
d,  e,  f,g,h,i.  Now,  suppose  that  part  of  the  earth's  crust, 
which  is  represented  in  the  frontispiece  to  be  subjected,  in  vari- 
ous places,  to  a  long  series  of  upheaving  and  depressing  move- 
ments ;  the  beds  A  will,  here  and  there,  be  partially  upraised 
and  converted  into  dry  land,  but  the  hypogene  rocks  C,  D,  al- 
though brought  up  nearer  to  the  surface,  will  still,  very  probably, 
remain  hidden  from  sight.  Let  a  second  period  elapse,  and  the 
rocks  A  may  be  raised  in  some  countries  to  a  height  of  several 
thousand  feet ;  and  still  the  rocks  C  and  D  may  be  almost  every 
where  hidden.  During  a  third  period,  when  the  stratified  form- 
ations A  have  been  laid  dry  over  large  continental  areas,  and 
have  reached  the  summits  of  some  Alpine  chains,  the  hypogene 
formations  C  D  may  also  be  forced  up  and  exposed  to  view  above 
the  level  of  the  ocean  by  similar  causes  ;  but  they  will  rank  no 
longer  as  modern  rocks,  the  geologist  being  already  acquainted 
with  newer  groups,  both  fossiliferous  and  volcanic.  The  student 
will  also  perceive  how  impossible  it  may  then  be  to  prove  that 
the  strata  C  became  metamorphic  at  the  period  of  the  deposition 
of  A,  and  how  difficult  not  to  exaggerate  the  antiquity  of  C  as  a 
series  of  metamorphic  rocks,  when  the  remote  period  of  their 
deposition  has  been  ascertained,  and  the  comparatively  modern 
era  of  their  crystallization  remains  uncertain. 

Order  of  succession  in  Metamorphic  rocks. — There  is  no 
universal  and  invariable  order  of  superposition  in  metamorphic 
rocks,  although  a  particular  arrangement  may  prevail  through- 
out countries  of  great  extent,  for  the  same  reason  that  it  is  trace- 
able in  those  sedimentary  formations  from  which  crystalline 
strata  are  derived.  Thus,  for  example,  we  have  seen  that  in  the 
Apennines,  near  Carrara,  the  descending  series,  where  it  is 


300  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Chronological  Relations  of  Metamorphic  Rocks. 

metamorphic,  consists  of,  1st,  saccharine  marble;  2dly,  talcose- 
schist ;  and  3dly,  of  quartz  rock  and  gneiss ;  where  unaltered, 
of,  1st,  fossiliferous  limestone;  2dly,  shale;  and  3dly,  sand- 
stone. 

But  if  we  investigate  different  mountain  chains,  we  find  gneiss, 
mica-schist,  hornblende-schist,  chlorite-schist,  hypogene  lime- 
stone, and  other  rocks,  succeeding  each  other,  and  alternating 
with  each  other,  in  every  possible  order.  It  is,  indeed,  more 
common  to  meet  with  some  variety  of  clay-slate  forming  the 
uppermost  member  of  a  metamorphic  series  than  any  other  rock  ; 
but  this  fact  by  no  means  implies,  as  some  have  imagined,  that 
all  clay-slates  were  formed  at  the  close  of  an  imaginary  period, 
when  the  deposition  of  the  crystalline  strata  gave  way  to  that  of 
ordinary  sedimentary  deposits.  Such  clay-slates,  in  fact,  are 
variable  in  composition,  and  sometimes  alternate  with  fossilife- 
rous strata,  so  that  they  may  be  said  to  belong  almost  equally  to 
the  sedimentary  and  metamorphic  order  of  rocks.  It  is  proba- 
ble that  had  they  been  subjected  to  more  intense  plutonic  action, 
they  would  have  been  transformed  into  hornblende  schist,  foliated 
chlorite-schist,  scaly  talcose-schist,  mica-schist,  or  other  more 
perfectly  crystalline  rocks,  such  as  are  usually  associated  with 
gneiss. 

Uniformity  of  mineral  character  in  Hypogene  rocks. — Hum- 
boldt  has  emphatically  remarked  that  when  we  pass  to  another 
hemisphere,  we  see  new  forms  of  animals  and  plants,  and  even 
new  constellations  in  the  heavens ;  but  in  the  rocks  we  still  re- 
cognize our  old  acquaintances, — the  same  granite,  the  same 
gneiss,  the  same  micaceous  schist,  quartz  rock,  and  the  rest.  It 
is  certainly  true  that  there  is  a  great  and  striking  general  resem- 
blance in  the  principal  kinds  of  hypogene  rocks,  although  of  very 
different  ages  and  countries ;  but  it  has  been  shown  that  each  of 
these  are,  in  fact,  geological  families  of  rocks,  and  not  definite 
mineral  compounds.  They  are  much  more  uniform  in  aspect 
than  sedimentary  strata,  because  these  last  are  often  composed 
of  fragments  varying  greatly  in  form,  size,  and  colour,  and  con- 
tain fossils  of  different  shapes  and  mineral  composition,  and  ac- 
quire a  variety  of  tints  from  the  mixture  of  various  kinds  of  sedi- 
ment. The  materials  of  such  strata,  if  melted,  and  made  to 
crystallize,  would  be  subject  to  chemical  laws,  simple  and 
uniform  in  their  action,  the  same  in  every  climate,  and  wholly 
undisturbed  by  mechanical  and  organic  causes. 

Nevertheless,  it  would  be  a  great  error  to  assume  that  the 
hypogene  rocks,  considered  as  aggregates  of  simple  minerals, 
are  really  more  homogeneous  in  their  composition  than  the 


PART  II.     CHAPTER  XXV.  301 


Hypogene  Rocks,  why  less  calcareous. 


several  members  of  the  sedimentary  series.  In  the  first  place, 
different  assemblages  of  hypogene  rocks  occur  in  different  coun- 
tries ;  and  secondly,  in  any  one  district,  the  rocks  which  pass 
under  the  same  name  are  often  extremely  variable  in  their  com- 
ponent ingredients,  or  at  least  in  the  proportions  in  which  each 
of  these  are  present.  Thus,  for  example,  gneiss  and  mica- 
schist,  so  abundant  in  the  Grampians,  are  wanting  in  Cumber- 
land, Wales,  and  Cornwall ;  in  parts  of  the  Swiss  and  Italian 
Alps,  the  gneiss  and  granite  are  talcose,  and  not  micaceous,  as 
in  Scotland ;  hornblende  prevails  in  the  granite  of  Scotland — 
schorl  in  that  of  Cornwall — albite  in  the  plutonic  rocks  of  the 
Andes — common  felspar  in  those1  of  Europe.  In  one  part  of 
Scotland,  the  mica-schist  is  full  of  garnets;  in  another,  it  is 
wholly  devoid  of  them ;  while  in  South  America,  according  to 
Mr.  Darwin,  it  is  the  gneiss,  and  not  the  mica-schist,  which  is 
most  commonly  garnetiferous.  And  not  only  do  the  propor- 
tional quantities  of  felspar,  quartz,  mica,  hornblende,  and  other 
minerals,  vary  in  hypogene  rocks  bearing  the  same  name ;  but, 
what  is  still  more  important,  the  ingredients,  as  we  have  seen, 
of  the  same  simple  mineral  are  not  always  constant,  (p.  92., 
and  table,  p.  102.) 

The  Metamorphic  strata,  why  less  calcareous  than  the  fos- 
siliferous. — It  has  been  remarked,  that  the  quantity  of  calcare- 
ous matter  in  metamorphic  strata,  or,  indeed,  in  the  hypogene 
formations  generally,  is  far  less  than  in  fossiliferous  deposits. 
Thus  the  crystalline  schists  of  the  Grampians  in  Scotland,  con- 
sisting of  gneiss,  mica-schist,  hornblende-schist,  and  other  rocks, 
many  thousands  of  yards  in  thickness,  contain  an  exceedingly 
small  proportion  of  interstratified  calcareous  beds,  although 
these  have  been  the  objects  of  careful  search  for  economical  pur- 
poses. Yet  limestone  is  not  wanting  in  the  Grampians,  and  it  is 
associated  sometimes  with  gneiss,  sometimes  with  mica-schist, 
and  in  other  places  with  other  members  of  the  metamorphic 
series.  But  where  limestone  occurs  abundantly,  as  at  Carrara, 
and  in  parts  of  the  Alps,  in  connexion  with  hypogene  rocks,  it 
usually  forms  one  of  the  superior  members  of  the  crystalline 
group. 

The  scarcity,  then,  of  carbonate  of  lime  in  the  plutonic  and 
metamorphic  rocks  generally,  seems  to  be  the  result  of  some 
general  cause.  So  long  as  the  hypogene  rocks  were  believed  to 
have  originated  antecedently  to  the  creation  of  organic  beings,  it 
was  easy  to  impute  the  absence  of  lime  to  the  non-existence  of 
those  mollusca  and  zoophytes  by  which  shells  and  corals  are 
secreted  ;  but  when  we  ascribe  the  crystalline  formations  to  plu- 
Aa 


302  LYELL'S  ELEMENTS  OF  GEOLOGY. 

Scarcity  of  Lime  in  Metamorphic  Rocks. 

tonic  action,  it  is  natural  to  inquire  whether  this  action  itself  may 
not  tend  to  expel  carbonic  acid  and  lime  from  the  materials 
which  it  reduces  to  fusion  or  semi-fusion.  Although  we  cannot 
descend  into  the  subterranean  regions  where  volcanic  heat  is 
developed,  we  can  observe  in  regions  of  spent  volcanos,  such  as 
Auvergne  and  Tuscany,  hundreds  of  springs  both  cold  and  ther- 
mal, flowing  out  from  granite  and  other  rocks,  and  having  their 
waters  plentifully  charged  with  carbonate  of  lime.  The  quan- 
tity of  calcareous  matter  which  thqse  springs  transfer,  in  the 
course  of  ages,  from  the  lower  parts  of  the  earth's  crust  to  the 
superior  or  newly  formed  parts  of  the  same,  must  be  con- 
siderable.* 

If  the  quantity  of  siliceous  and  aluminous  ingredients  brought 
up  by  such  springs  were  great,  instead  of  being  utterly  insigni- 
ficant, it  might  be  contended  that  the  mineral  matter  thus  expelled 
implies  simply  the  decomposition  of  ordinary  subterranean  rocks ; 
but  the  prodigious  excess  of  carbonate  of  lime  over  every  other 
element  must,  in  the  course  of  time,  cause  the  crust  of  the  earth 
below  to  be  almost  entirely  deprived  of  its  calcareous  constitu- 
ents, while  we  know  that  the  same  action  imparts  to  newer  depo- 
sits, ever  forming  in  seas  and  lakes,  an  excess  of  carbonate  of 
lime.  Calcareous  matter  is  poured  into  these  lakes  and  the 
ocean  by  a  thousand  springs  and  rivers ;  so  that  part  of  almost 
every  new  calcareous  rock  chemically  precipitated,  and  of  many 
reefs  of  shelly  and  coralline  stone,  must  be  derived  from  mineral 
matter  subtracted  by  plutonic  agency,  and  driven  up  by  gas  and 
steam  from  fused  dnd  heated  rocks  in  the  bowels  of  the  earth. 

Not  only  carbonate  of  lime,  but  also  free  carbonic  acid  gas  is 
given  off  plentifully  from  the  soil  and  crevices  of  rocks  in 
regions  of  active  and  spent  volcanos,  as  near  Naples,  and  in 
Auvergne.  By  this  process,  fossil  shells  or  corals  may  often 
lose  their  carbonic  acid,  and  the  residual  lime  may  enter  into  the 
composition  of  augite,  hornblende,  garnet,  and  other  hypogene 
minerals.  That  the  removal  of  the  calcareous  matter  of  fossil 
shells  is  of  frequent  occurrence,  is  proved  by  the  fact  of  such 
organic  remains  being  often  replaced  by  silex  or  other  minerals, 
and  sometimes  by  the  space  once  occupied  by  the  fossil  being 
left  empty,  or  only  marked  by  a  faint  impression.  We  ought 
not  indeed  to  marvel  at  the  general  absence  of  organic  remains 
from  the  crystalline  strata,  when  we  bear  in  mind  how  often  fos- 
sils are  obliterated,  wholly  or  in  part,  even  in  tertiary  forma- 
tions— how  often  vast  masses  of  sandstone  and  shale,  of  different 

*  See  Principles  of  Geology,  Index,  "  Calcareous  Springs." 


PART  II.     CHAPTER  XXV.  303 

Scarcity  of  Lime  in  Metamorphic  Rocka. 

ages,  and  thousands  of  feet  thick,  are  devoid  of  fossils — how 
certain  strata  may  first  have  been  deprived  of  a  portion  of  their 
fossils  when  they  became  semi-crystalline,  or  assumed  the  tran- 
sition state  of  Werner — and  how  the  remaining  organic  remains 
may  have  been  effaced  when  they  were  rendered  metamorphic. 
Some  rocks  of  the  last-mentioned  class,  moreover,  must  have 
been  exposed  again  and  again  to  renewed  plutonic  action. 


INDEX. 


ABERDEENSHIRE,  granite  of,  123. 
Acephalous  mollusca,  44. 

Acrodus  nobilis,  226. 

Actinoceras  Simmsii,  268. 

Actinolite,  102.  134. 

Agassiz,  on  fossil  fish,  180.  225,  226 
237.  241.  246.  263. 

Age    of  aqueous    strata,  how  deter- 
mined, 159. 

,  of  volcanic  rocks,  271. 

,  of  the  plutonic  rocks,  281. 

,  of  the  metamorphic  rocks,  293. 

Airdnamurchan,  trap  veins  in.  104. 

Albite,  102. 

Alluvium  described,  83. 

,  passes  into  regular  strata,  84. 

,  marine,  85. 

Alps,  reversed  position  of  strata  in,  73. 
297. 

,  curved  strata  of,  73,  74. 

,  metamorphic  rocks  of  the,  287. 

296. 

Altered  rocks,  21.  107.  123.  140.  143. 
286.  293. 

Alternations  of  coarse  and  fine  strata, 
how  formed,  16.  29. 

,  of  marine  and  freshwater  forma- 
tions, 49. 

Alumine  in  rocks,  how  to  detect,  26. 

Amblyrhynchus  cristalus,  229. 

America,  Recent  and  Tertiary  strata 
of,  171. 

Silurian  strata  in,  268. 

Amici,  Professor,  on  recent  Charae,  48. 

Ammonites,  figures  of,  189. 220. 238. 246. 

Ampelite,  134. 

Amphibolite,  93.  134. 

Ampullaria  glauca,  46. 

Amygdaloid  described,  96. 

Ananchytes  ovatus,  185. 

Ancylus  elegans,  45. 

Andes,  geological  structure  of,  284, 285. 

,  tertiary  plutonic  rocks  of,  286. 

Anglesea,  rocks  altered  by  a  dike  in, 

Anodonta,  figures  of,  44. 
Anoplotherium,  180. 
Ansted,  Mr.,  on  Cambrian  fossils,  270. 
Anticlinal  line  explained,  66.  72. 
Antrim,  rocks  altered  by  dikes  in,  107. 

,  on  age  of  trap  rocks  of,  278. 

Apennines,  age  of  metamorphic  rocks 

of,  295. 

Apes,  fossil,  180. 
Apiocrinites  rotundus,  216. 
Aa* 


Aqueous  rocks  described,  15.  159. 

Arbroath,  section  from,  to  the  Gram- 
pians", 65. 

Arenaceous  rocks  described,  25. 

Argillaceous  rocks  described,  26.  133. 

Arran,  dikes  in,  104.  , 

,  geology  of,  291,  292. 

,  section  of,  292. 

Arthur's  seat,  strata  altered  in,  109. 

Asapkus,  figures  of,  266,  267. 

Ashby,  faults  in  coal-field  of,  81. 

Ashes,  volcanic,  hollows  filled  up  by, 
30,  31. 

,  wide  dispersion  of,  273,  274. 

Astarte,  176. 

Atlantis,  210. 

Auch,  ape  fossil  near,  180. 

Augite  and  hornblende,  analogy  of,  92, 
9o. 

,  analysis  of,  102. 

Augite  rock,  99. 

Augitic  porphyry,  99. 

Auricula,  45. 

Autreppe,  unconformable  strata,  74,  75. 

Auvergne,  volcanos  of,  18.  91.  276. 

,  freshwater  strata  of,  43. 

Auvergne,  rocks  decomposed  by  car- 
bonic acid  in,  147. 

,  tertiary  red  sandstone  of,  243. 

,  calcareous  springs  of,  302. 

Amcula,  figures  of,  224.  238. 

JBacillaria  in  tripoli,  39. 

Baculites,  figures  of,  184. 

Bakewell,  Mr.,on  structure  ofrocks,  139. 

Bala  limestone,  270. 

Baltic,  rocks  drifted  by  ice  in,  87. 

Basalt  described,  95.  100. 

— ,  columnar,  110. 

— ,  sandstone  rendered  columnar  by, 

140. 
and  trachyte,  relative  position  of, 

274,  275. 

Basin,  or  trough,  described,  66. 
Basset,  term  explained,  72. 
Bayfield,  Captain,  on  transportation  of 

rocks  by  ice,  72. 

— ,  on  worn  limestone  pillars,  194. 
Beaumont,  M.  E.  de,  on  lias  of  the  Vos- 

ges,  225. 
,  on  metamorphic  rocks  in  the  Alps, 

287.  298. 
Beck,  Dr.,  on  recent  and  tertiary  fossil 

shells,  167. 

— ,  on  classification  of  tertiary  strata, 

168. 

(305) 


306 


INDEX. 


Beck,  Dr.,  on  proportion  of  species  to 

genera  in  different  latitudes,  168. 

,  on  stones  carried  by  sea- weed,  188. 

,  on  Graptolites,  268. 

Belemnites,  figures  of,  184.  220. 
Bellerophon  costatus,  254. 
Berenicea  diluviana,  217. 
Berger,  Dr.,  on  dikes  in  chalk  of  An- 
trim, 107.  278. 

Bergmann  on  trap  rocks,  89. 
Bernese  Alps,  sections  in,  298. 
Berthier,  on  augite,  93. 
Bertrich-Baden,  globular  structure  in 

basaltic  columns  at,  113. 
Berwickshire,  curved  strata  on  coast  of, 

66. 

Biggleswade,  section  near,  32. 
Bilin,  tripoli  of,  composed  of  infusoria, 

39. 

Binstead,  fossils  of,  180. 
Birds,  fossil,  in  Wealden,  203. 
Bischoff,  Professor,  cited,  146. 
Blainville,  on  number  of  genera  of 

mollusca,  43,44. 
Boase,  Dr.,  cited,  122.  149. 
Boblaye,  M.,  on  geology  of  the  Morea, 

199:278. 

Bog  iron-ore,  infusoria  fossil  in,  40,  41. 
Bonpland,  cited,  251. 
Bothnia,  Gulf  of,  proofs  of  rise  of  land 

in,  63. 

Boue,  Mr.,  bis  classification  of  rocks,  157. 
,  on  metamorphic  rocks,  295,  296. 

298. 

Bradford  clay,  fossils  of,  216. 
Brash  described,  83. 
Bray,  valley  of,  182.  210. 
Breccia,  volcanic,  98. 
Bridgnorth,  tertiary  strata  of,  173. 
Brongniart,  M.  Alex.,  on  vertical  trees 

in  coal  strata,  258. 
Brongniart,  M.  Ad.,  on  fossil  coal  plants, 

247,  248,  249,  250. 

,  on  climate  of  carboniferous  pe- 
riod, 254,  255. 
Bronn,  Professor,  on  fossils  of  upper 

New  Red  sandstone,  237,  238. 

,  on  Orthocerata,  268. 

Brora  coal-field,  223. 

,  granite  of,  291. 

Buckland,  Dr.,  on  changes  caused  by 

dikes,  108. 

,  on  coprolites,  187. 

,  on  origin  of  flint  in  chalk,  190. 

,  on  fossils  of  Oolite,  217.  222. 

,  on  dirt-bed  in  Portland,  206,  207. 

,  on  Ichthyodorulites,  226,  227. 

,  on  saurians  of  the  Lias,  228,  229, 

230. 

,  on  the  New  Red  sandstone,  236. 

,  on  fossil  footsteps,  238,  239. 

,  on  fossil  coal  plants,  252.  259. 

,  his   classification   of   secondary 

rocks,  265. 

Budenheim  limestone,*  43. 
Bulimus  lubricus,  46. 


Bunter  sandstein  and  fossils,  237. 
Burdiehouse  fossils,  245. 

Caer  Caradoc,  trap-tuffs  of,  280. 

Cairo,  strata  formed  by  the  Nile  at,  16. 

Caithness,  fossil  fish  of,  260. 

Calamites,  figures  of,  251. 

Calcareous  rocks  described,  26,  27. 

Calcareous  springs,  action  of,  302. 

Caldcleugh,  Mr.,  on  eruption  of  Cose- 
guina,  274. 

Calimene  Blumenbachii,  266. 

Cambrian  rocka  and  fossils,  264,  265. 
270. 

,  volcanic  rocks,  280. 

Campagna  di  Roma,  tuffs  of,  276. 

Cantal,  volcanic  rocks  of,  277. 

Cape  Wrath,  granite  veins  of,  126. 

Caradoc  sandstone,  267. 

Carbonate  of  lime,  102. 

,  why  least  in  oldest  rocks,  301. 

Carbonic  acid,  in  water  of  sediment  in 
delta  of  Ganges,  58. 

,  rocks  decomposed  by,  146. 

Carboniferous  limestone  and  fossils,  253. 

Carboniferous  period,  rocks  of,  243. 

,  climate  of,  254. 

,  trap  rocks  of,  279. 

,  plutonic  rocks  of,  288. 

,  subsidence  in,  257. 

.     See  Coal. 

Carpathians,  cretaceous  rocks  of,  193. 

Carrara  marble,  altered  oolite,  295. 

Caryophyllia  annularis,  215. 

Castrogiovanni,  bent  strata  near,  73. 

Casts  of  shells,  how  formed,  55. 

Catenipora  esckaroides,  267. 

Catillus  cuvieri,  182,  183. 

Caulopteris  prirruzva,  248. 

Cautley,  Capt,  on  fossil  monkey,  180. 

Celsius  on  rise  of  land  in  Sweden,  63. 

Cementing  together  of  particles  in  stra- 
ta, 51. 

Cephalaspis  Lyellii,  263. 

Ceratites,  23^. 

Cerithium  cinctum,  46. 

Chceropotamus,  180. 

Chain-corals,  267,  268. 

Chalk,  white,  composition,  &c.,  27. 182. 

,  fossils  of,  41. 182, 183. 186, 187. 196. 

,  origin  of  the,  186. 

,  pebbles  in,  187. 

,  its  extent,  191. 

,  external  configuration  of,  193. 

,  needles  and  escarpments  of,  194. 

,  greatest  height  of,  in  England,  195. 

Chalk-flints,  infusoria  in,  42. 

,  origin  of,  190. 

Chalk  formation,  its-marine  origin,  181. 

,  subdivisions  of,  181. 

,  fossils  of,  182.  186,  187.  196, 197. 

,  geographical  extent,  191. 

,  difference  of,  in  north  and  south 

of  Europe,  195. 

,  map  of,  in  S.  of  France,  196. 

,  altered  by  granite,  145. 


INDEX. 


307 


Chalk  formation,  covered  by  granite, 

near  Meissen,  291. 
,  alternating    with    volcanic    tuff 

277,  278.    See  Cretaceous. 
Champoleon,  junction  of  granite  and 

secondary  rocks  near,  287. 
Chares,  fossil  in  freshwater  strata  (see 

figures),  47,  48. 

Charlesworth,  Mr.,  on  the  crag,  1#3. 
Charpentier,    M.,  on    trap    rocks    of 

Pyrenees,  278. 

Cheese-grotto,  at  Bertrich-Baden,  113. 
Chemical  deposits,  50. 
Cheshire,  ripple-marked  sandstone  from 

(see  fig.  6).,  33. 
Chevalier,  M.,  on  bent  coal  strata  near 

Mons,  69. 
Chiastolite,  102. 
Chiastolite-slate,  134. 
Chimcera  monslrosa,  226. 
Chimney,    the,  basaltic    dike    m    St. 

Helena,  111. 
Chirotherium,  238. 
Chlorite,  composition  of,  102. 
Chlorite-schist  described,  134. 
Christiania,  dikes  near,  105,  106. 

,  granitic  rocks  of,  128, 129. 

,  passage  of  trap  into  granitic  rocks, 

near,  123. 
,  porphyry  conformable   to  strata 

near,  129,  130. 

,  rocks  altered  by  granite,  near,  143. 

,  tertiary  strata,  near,  171. 

Cidaris  coronata,  219. 

Ciply,  cretaceous  rocks  at,  189. 

Classification  of  rocks,  principles  on 

which  it  is  founded,  15.  25. 
,  of  the  fbssiliferous  rocks,    157. 

163.  264. 

Clay  described,  25. 
Clay-slate  described,  133. 
,  lamination  of,  in  the  Pyrenees, 

137. 
,  position  of,  .m  the  metamorphic 

series,  300. 

Claystone,  and  Claystone  porphyry,  100. 
Cleavage  of  rocks,  137. 
Climate  of  carboniferous  period,  254. 
Clinkstone,  100.  137. 
Club-mosses,  250. 
Coal,  vegetable  origin  of,  42.  244. 

,  fossils  of  the,  244. 

,  strata,  origin  of,  244.  256. 

,  on  vertical  trees  in,  42.  257. 

,  isolated  patches  of,  256. 

,  rate  of  deposition  of,  258.  261. 

,  zigzag  flexures  of,  near  Mons,  68. 

.    See  Carboniferous. 

Coal-pipes,  259. 
Coalbrook-dale  coal-field,  245. 
CockfieJd  Fell,  coal  turned  into  soot  by 

dike  at,  108. 
Columnar  structure  in  rocks,  110.  119. 

140. 

,  in  ice,  140. 

CoLumnarici  oblonga,  215. 


Compact  felspar,  100. 

Concretionary  structure  in  rocks,  53, 

o4. 

Cones  and  craters,  how  formed,  91. 
Conglomerates  described,  26. 
— — ,  vertical,  in  Scotland,  &c.,  64. 

,  of  New  Red  sandstone.  240. 

Coniferse,  fossil,  56.  251.  255. 
Consolidation  of  strata,  50. 
Conybeare,  Rev.  W.  D.,  on  faults,  78. 82. 
,  on  changes  caused  by  dikes,  108. 

on  the  Chalk  formation,  181. 

on  the  Plesiosaurus,  227. 

on  the  Oolite  and  Lias,  224.  232. 

on  the  New  Red  sandstone,  236. 

on  the  Coal  strata,  244. 
Coprolites  of  the  Chalk,  187. 
Coral  islands,  changes  of  level  in,  64. 
Coral  rag,  fossils  of  the,  215.  218,  219. 
Coral  reefs,  great  extent  of,  191. 
Corals,  fossil,  175.  214,  215. 
Corbula  alata,  201. 
Cornean,  100. 

Cornwall,  structure  of  granite  of,  119, 
120. 

,  granite  veins  in,  127.  145. 

Coseguina,  volcanic  eruption  of,  273. 
Cotta,  Dr.,  on  granite  of  Weinbohla, 

291. 
Crag  formation,  and  its  fossils,  174. 177. 

,  division  of,  into  red  and  coralline, 

174. 

,  may  all  belong  to  one    period, 

176,  177. 

,  its  relative  position,  178. 

Craigleith  fossil  trees,  56.  259. 
Crania,  figures  of,  37.  183. 
Crassatella  sidcata,  179. 
Craters,  volcanic,  how  formed,  91. 
Craven  fault,  78. 
Cretaceous  period,  181. 

,  volcanic  rocks  of,  277. 

,  plutonic  rocks  of,  286. 

.     See  Chalk. 

Crop  out,  term  explained,  72. 
Cropthorne,  tertiary  deposits  at,  173. 
Cuba,  tertiary  strata  in,  172. 
Curved  strata,  65,  66. 

,  experiments  to  illustrate,  67. 

,  origin  of,  68. 

Cutch,  changes  caused  by  earthquakes 

in,  208. 

Cyalhea  glauca,  249. 
Cyatliocrinites  planus,  242. 
Cyclas  obovata,  44. 
Cyprcea  coccinelloides,  175. 
Cypris,  fossil  in  freshwater  strata,  47. 

,  figures  of,  202.  245. 

Cyrena  trigoluna,  44. 
Cylherince  of  the  Chalk,  41. 

Dartmoor  granite,  145.  288. 
Daubeny,  Dr.,  on  the  Solfatara,  147. 
Dax,  chalk  near,  197. 
,  chalk  and  volcanic  tuff  alternat- 
ing, near,  277. 


308 


INDEX. 


Darwin,  Mr.  C.,  on  gradual  rise  of  parts 
of  S.  America,  63. 

,  on  coral  islands,  64. 

,  on  formation  of  mould,  83. 

,  on  shivering  of  rocks  in  Chili  by 

earthquakes,  83. 

,  on  transportation  of  rocks  by  ice, 

87,88. 

,  on  slaty  structure  in  refuse  of 

gold  mine,  142. 

,  on  structure  of  Andes,  284. 

,  on  recent  strata  near  Lima,  171. 

,  on  origin  of  chalky  mud  in  Pa- 
cific, 186. 

,  on  drifting  of  stones  in  roots  of 

trees,  187. 

,  on  stones  attached  to  sea- weed, 

188. 

,  on  living  saurian  of  the  Galapa- 
gos, 229. 

,  on  subsidence  in  Pacific,  233. 

De  la  Beche,  Mr.,  on  calcareous  no- 
dules in  Lias,  53. 

— -,  on  rocks  altered  by  granite,  145. 

,  on  dirt-bed  in  Portland,  206,  207. 

,  on  saurians  of  the  Lias,  228.  231. 

,  on  trap  rocks  of  New  Red  sand- 
stone, 278. 

,  on  metamorphic  rocks,  296. 

Delta  of  Indus,  recent  changes  in,  208. 

,  the  Niger,  its  size,  209,  210. 

Deluge,  fossils  attributed  by  some  to,  17. 

Denmark,  cretaceous  coral  reef  in,  188. 

Denudation  defined,  79. 

,  its  great  amount,  80. 

,  valleys  of,  80. 

,  on  a  great  scale  in  Ross-shire,  81. 

,  proofs  of,  from  levelled  surface  of 

districts  where  great  faults  occur,  81. 

,  connexion  of  alluvial  formations, 

and,  82. 

,  proofs  of  from  trap-dikes,  115. 

Deshayes,  M.,  his  identifications  of  re- 
cent and  fossil  shells,  165,  166,  167. 
177. 

Deoxydation  of  mineral  waters  by  or- 
ganic matter,  58. 

Devonshire,  trap  rocks  of,  278. 

Diagonal  stratification  explained,  31. 

Diallage,  102. 

Diallage  rocks,  100. 

Diceras  arietina,  219. 

Diceras  limestone,  219. 

Didelphys,  fossil,  222. 

Dikes,  volcanic,  described,  19.  103. 

• ,  more  crystalline  in  the  centre,  105. 

,  fragments  of  subjacent  rocks  in, 

106. 

,  changes  caused  by,  19.21. 104. 107. 

,  granitic,  125. 

Diluvium,  ancient  alluviums  called,  85. 

Dimyary  mollusca,  44. 

Dionte  and  dioritic  porphyry,  100. 

Dip,  term  explained,  69. 

,  how  to  measure,  70. 

,  reversed,  how  caused,  73. 


Dirt-bed  in  isle  of  Portland,  205. 
Dolerite  described,  96.  100. 
Dolomite  described,  28. 
Dolomitic  conglomerate,  fossils  of,  240, 
Domite,  100. 

Drift-wood  of  American  rivers,  260. 
Dudley,  altered  coal  shales  of,  145. 
Dufrenoy,  M.,  on  rocks  altered  by  gra- 
nite, 145. 

Earth's  crust,  term  explained,  14. 
,  composed  of  distinct  substances, 

13. 

,  its  successive  formation,  13. 

,  arrangement  of  its  materials,  14. 

,  not  increasing  in  thickness,  157. 

Echini  from  the  Chalk,  parasitic  fossils 

on,  37. 

Edinburgh  coal-field,  fossils  of,  245,  246. 
Ehrenberg,  Professor,  on  infusoria,  39. 

41,  42. 

Eifel,  volcanic  rocks  of  the,  276. 
Elbeuf,  needles  and  grooved  pillars  of 

chalk  at,  194. 

Elevation  of  land,  gradual,  proofs  of,  62. 
Encrinites  fossil  in  Oolite,  216,  217. 
Endosiphonites  carinatus,  270. 
England,  tertiary  strata  of,  172.  180. 
Enstone,  fossil  bone  from,  222. 
Eocene,  term  whence  derived,  166. 

strata,  in  England,  178. 

volcanic  rocks,  276. 

Epidote,  102. 

Equisetacece,  237.  247.  250.  255. 

Erratic  blocks,  distribution  of,  86. 

,  transported  by  ice,  86. 

Escarpments  of  oolites,  &c.,  234. 

Esckara  dislicJia,  185. 

Estuary  deposits,  16. 

Etna,  lavas,  tuffs,  and  dikes  of,  273. 276. 

Eunomia  radiata,  215. 

Euphorbiacece,  252. 

Euphotide,  100. 

Eurite  and  euritic  porphyry  described, 

122.134. 
Exogyra  butta,  201. 

Falconer,  Dr.,  on  fossil  monkey,  180. 
False  stratification  explained,  32. 
Fascicularia  aurantium,  176. 
Faults  described,  75. 
cause    apparent  alternations  of 

strata,  77. 

,  great  amount  of  some,  78.  82. 

,  origin  of,  77,  78. 

,  grooved  surfaces  of,  78. 

,  denudation  proved  by,  81,  82. 

Faxoe  limestone  and  its  fossils,  188. 

Felspar,  varieties  of,  92.  102. 

,  its  decomposition  affords  silex  in 

solution,  59. 
Felspar-porphyry,  100. 
Findheim.land  shells  in  limestone  of,  43. 
Fish  killed   by  submarine    eruptions, 

floods,  &c.,  231. 
Fissures,  polished  surfaces  of,  75.  78. 


INDEX. 


Fit  ton,   Dr.,  on  the  Green-sand,   181. 

191,  192. 

• ,  on  the  Maestricht  beds,  189. 

,  on  the  Wealden  strata,  200,  201. 

205.  209. 

,  on  the  Portland  dirt-bed,  206, 207. 

Fleming,  Dr.,  on  fossil  fish  of  Old  Red 

sandstone,  263. 

,  on  trap  rocks,  279,  280. 

Flint,  sponge  fossil  in,  185. 

,  in  chalk,  its  origin,  190. 

Flotz  rocks  of  Werner,  153. 
Footsteps,  fossil,  238,  239. 
Foraminifera  of  the  Chalk,  41. 
Forfarshire,  geology  of,  65.  262. 

,  decomposition  of  rocks  in,  242. 

Formation,  term  explained,  16. 
Formations,  fossiliferous,  arrangement 

of,  157.  163. 

Forth  coal-field,  trap  rocks  of,  279. 
Fortis,  on  columnar  basalt,  112. 
Fossil,  term  defined,  16. 
Fossils  in  stratified  rocks,  16. 

,  height  at  which  they  are  found,  17. 

,  their  arrangement  in  strata,  35. 

,  parasitic,  prove  gradual  deposi- 
tion, 36. 

,  freshwater  and  marine,  42. 

,  their  absence  in  some  rocks,  how 

explained,  52.  302. 

,  mineralization  of,  55. 

,  casts  and  impressions    of)    how 

formed,  55. 
Fossiliferous  strata,  conversion  of  into 

metamorphic  rocks,  295. 
— — ,  why  most  calcareous,  301. 
Fournet,  M.,  on  disintegration  of  rocks, 

Fox,  Mr.  R.  W.,  his  experiments  on 
lamination,  142. 

Fox,  Rev.W.D.,  on  fossil  mammalia,  180. 

Freshwater  formations,  how  distin- 
guished from  marine,  42. 

,  land  shells  numerous  in,  43. 

,  fossils  numerous,  but  species  few 

in,  44. 

,  figures  of  shells  most  common  in, 

44,  45,  46. 

,  Cypris  fossil  in,  47. 

,  Charae  fossil  in,  48. 

,  vertebrated  animals  in,  48. 

,  alternating  with  marine,  causes 

of,  49. 

Fresh  water  strata  of  the  Coal,  244.256. 

Frontispiece  described,  22. 

Fusus  contrarius,  175. 

Gabbro,  100. 

Gaillonella,  fossil  in  tripoli,  39.  41. 

Galapagos  islands,  living  marine  reptile 

of,  229. 

Ganges  river,  deposits  in  estuary  of,  16. 
Garnet,  102. 

. ,  in  altered  rocks,  107.  301. 

Gases,  subterranean,  rocks  altered  by, 

146, 147. 


Gault,  191. 

Gavarnie,  curved  strata  near,  74. 

Geology  defined,  13. 

Gestellihorn,  section  at  base  of  the,  298. 

Giant's  Cause  way,  volcanic  rocks  of,  19. 

Glen  Tilt,  junction   of  granite,  schist, 

and  limestone  in,  123.  125. 
Globular  structure,  110. 
Gneiss  described,  132. 
Gold  mine,  slaty  structure  in  refuse  of, 

142. 

Goniatites  evolutus,  254. 
Goppert,  Prof,  his  experiments  on  fbs- 

silization  of  plants,  57. 
Gosforth,  tree  in  coal  strata  at,  56.  259. 
Graham  island,  116. 
Grampians,  vertical  conglomerates  in 

the,  64. 

,  section  from  to  the  sea,  65. 

,  dikes  of  granite  in,  125. 

,  decomposed  rocks  of,  242. 

,  rarity  of  limestones  in  the,  301. 

Granite,  of  igneous  origin,  20.  148. 

,  of  different  ages,  23.  126.  281. 

,  general    aspect,    structure,    and 

composition  of,  119. 

,  varieties  of,  120,  121. 

,  passage  from  trap  to,  123. 

,  analogy  in  composition  of  trachyte 

and,  123. 

,  veins,  123. 

,  finer  grained  in  veins,  126. 

,  isolated  masses  of,  128. 

,  whether  it  ever  overlies  fossilife- 
rous rocks,  129.  291. 
,  rocks  altered   by,   20.  143.  145. 

284.  289. 

,  on  the  most  ancient,  290. 

,  protrusion  of  solid,  291. 

,  of  Arran,  age  of,  291. 

.    See   also    Plutonic  rocks,  and 

Hypogene  rocks. 
Graphic  granite  described,  120. 
Graptolites,  268. 
Grateloup,    Dr.,    on    chalk   of  S.  of 

France,  197. 
Grauwacke,  term,  264. 

,  different  ages  of,  264. 

Graves,  M.,  on  valley  of  Bray,  210. 
Gray,  Mr.,  cited,  45. 
Greenland,  subsidence  of  part  of,  63. 
Green-sand  formation,  191. 

— ,  fossils  of,  192. 

— ,  its  origin,  192. 
Greenstone  described,  96.  100. 
Greystone,  100. 
Grit  defined,  26. 

Grypheea,  figures  of,  36.  218.  225. 
Gryphite  limestone,  225. 
Guadaloupe,  human  skeletons  of,  171. 
Guidoni,  M.,  on  altered  oolite,  295. 
Gusigny,  section  at,  75. 
Gypsum,  composition  of,  28. 
Gyrogonites,  48. 

Hall,  Sir  J.,  on  curved  strata,  67. 


310 


INDEX. 


Hail,  Capt.  B.,  on  dikes  in  Madeira,  103. 

,  on  granite  veins,  125. 

Hamiles  spinig-er,  192. 

Harwich,  section  in  cliffs  at,  178. 

Hastings  Sand,  200.  208. 

Heat,  consolidating  effects  of,  55. 

Hebrides,  trap  rocks  of,  278. 

Heidelberg,  granites  of  different  apes 
at,  126. 

Helix  plebeium,  46. 

Henry,  experiments  of,  146. 

Henslow,  Prof,  on  changes  caused  by 
a  dike  in  Anglesea,  107. 

,  on  the  Portland  dirt-bed,  206. 

Herschel,  Sir  J.,  on  slaty  cleavage,  142. 

Hertfordshire  puddingstone,  52. 

Hewett,  Capt,  on  new  channel  in  Yar- 
mouth sands,  178. 

Hibbert,  Dr.,  on  fossils  of  the  Coal,  245. 

High  Teesdale,  garnets  in  altered  rock 
at,  107. 

— —*,  intrusion  of  trap  between  strata 
at,  110. 

Hildburghausen,  fossil  footsteps  at,  238. 

Hippurites,  figures  of,  196,  197. 

Hoffman,  on  agency  of  subterranean 
gases,  147. 

,  on  metamorphic  rocks,  295. 

Hoogly,  R.,  analysis  of  water  contained 
in  mud  of,  58. 

Hornblende,  92.  102. 

Hornblende-rock,  100.  135. 

Hornblende-schist  described,  133. 

Hornstone,  hornslone-porphyry,  100. 

Horner,  Mr.,  on  fossil  fish  in  Coal  stra- 
ta, 245,  246. 

,  on  the  Malvern  hills,  288. 

Hubbard,  Prof.,  on  granite  veins,  283. 

Hugi,  M.,  on  alternation  of  gneiss  and 
fossiliferous  rocks  in  Alps,  298. 

Humboldt  cited,  251. 

Hungary,  trachyte  of,  123. 

Hutton,  opinions  of,  153. 

Hutton,  Mr.,  on  fossil  coal  plants,  247. 
252.  259. 

Hybodus  relicutatus,  226. 

Hypersthene,  analysis  of,  102. 

Hypersthene  rock,  101. 

Hyppgene,  name  proposed  instead  of 
primary  for  the  crystalline  rocks,  24. 

,  rocks  described,  118.  134. 

,  must  be  old  before  they  reach  the 

surface,  298,  299. 

,  age  of,  how  determined,  282.  294. 

,  uniformity  of  mineral  character 

in,  300. 

,  why  less  calcareous  than  the  fos- 
siliferous, 301. 

.  See  also  Granite,  Plutonic  rocks, 

and  Metamorphic  rocks. 

Ice,  transportation  of  erratic  blocks  by, 
86,  87. 

,  columnar  structure  of,  140. 

Ichthyodorulites,  226.  262. 
Ichthyosaurus,  228. 


Iguanodon,  203.211. 
Inclined  and  vertical  stratification,  64 
Indus,  recent  changes  in  delta  of,  208. 
Infusoria  in  tripoli,  &c.,  39. 

,  figures  of,  40. 

Inkpen  Beacon,  195. 

Insects,  fossil,  221,  222. 

Inverted  position  of  strata,  how  caused, 

73. 

Ipswich,  section  near,  175. 
Ireland,  tertiary  strata  in,  172. 
Ischia,  tertiary  strata  in,  276. 
Iselten  Alp,  curved  strata  of  the,  74,  75. 
Isle  of  Bourbon,  eruptions  in,  283. 
Isle  of  Wight,  tertiary  strata  of,  43. 180. 

,  chalk  needles  of;  195. 

Isomorphism,  theory  of,  94. 

Jackson,  Col.,  on  columnar  structure  in 

ice,  140. 

Jointed  structure  of  rocks,  138. 
Jorullo,  volcanic  eruption  of,  283. 
Jungfrau,  section  on  the,  298. 
Jura,  section  of  structure  of  the,  71. 
,  Oolite  of  the,  214.  218.  233. 

Kander,  R.,  land  shells  in  delta  of,  43. 

Kaolin,  mineral  composition  of,  26. 

Kaup,  Prof,  on  fossil  footsteps,  239. 

Keilhau,  Prof,  on  Greenstone  dike,  106. 

,  on  granitic  rocks  of  Norway,  128. 

130.  144.  289. 

Kelloway  rock,  cementing  of  the  parti- 
cles of;  by  lime  derived  from  shells,  51. 

Keuper  sandstone,  fossils  of,  237. 

Kildonan  castle,  dike  near,  104. 

Killas  altered  by  granite,  145. 

Kimmeridge  clay,  221.  234. 

Labradorite,  92.  102. 

La  Coupe  d'Ayzac,  columnar  lava  of; 

Lakes,  arrangement  of  deposits  in,  15. 

Lamarck,  his  division  of  bivalve  mol- 
lusca,  44. 

Land,  proofs  of  the  elevation  and  sub- 
sidence of,  61.  63. 

Lander,  Mr.,  on  delta  of  Niger,  210. 

Land's  End,  granite  of,  119.  121. 

Land  shells,  numerous  in  freshwater 
formations,  43. 

,  drifted  by  rivers,  43. 

,  figures  of  genera  most  common 

in  strata,  46. 

Lartet,  M.,  on  fossil  ape,  180. 

Lateral  movements,  folding  of  strata 
by,  68. 

Lava  described,  91.  97. 

Lehman's  division  of  rocks,  152. 

Leibnitz,  theory  of,  156. 

Lepidodendra,  figures  of;  250. 

Lepidotus  figures  of,  202.  225. 

Leucite,  102. 

Lias,  mineral  character  of,  224,  225. 
— ,  fossils  of,— shells,  224.  231,— fish, 
225  ;- reptiles,  227;— plants,  232. 


INDEX. 


311 


Lias  and  Oolite,  origin  of,  232. 

,  valleys  and  escarpments  formed 

by,  234. 

,  calcareous  nodules  in,  53. 

,  volcanic  rocks  of  the,  278. 

,  plutonic  rocks  of  the,  287. 

Lima,  Recent  strata  near,  171. 
Lime  in  rocks,  how  to  detect,  27. 

,  whence  derived,  59. 

,  why  less  in  crystalline  rocks,  301. 

Limestones,  composition  of,  27.  36. 

,  deposited  by  springs,  50. 

,  in  coral  reefs  formed  by  zoophytes, 

50. 

Limnea  longiscata,  45. 
Lindley,  Mr.,  on  fossil  coal  plants,  247, 

248,  249.  252.  255.  259. 
,  on   destructibility    of  plants    in 

water,  255. 
Lipari  islands,  rocks  altered  by  gases 

in,  147. 

Llandeilo  formation,  267. 
Loam  described,  28. 
Loess  of  the  Rhine,  172. 

,  tuffs  interstratified  with,  276. 

Loire,  R.,  stratification  of  recent  mud 

of,  29. 

London  clay  and  its  fossils,  38.  178. 
Lonsdale,  Mr.,  on   microscopic  chalk 

fossils,  41. 

,  on  Stonesfield  slate,  222. 

Lons-le-Saulnier,  Lias  and  Oolite  of,  224. 
Louisiana,  submerged  trees  in,  261. 
Lower  New  Red  sandstone,  242. 
Ludlow  formation,  265. 
Lulworth  cove,  section  in,  206. 
Lumley  Den,  trap  rocks  in,  280. 
Lutschine,  valley  of,  curved  strata  in,  74. 
Lycopodiacete,  247.  250.  255. 
Lym-Fiord,  invaded  by  the  sea,  49. 
,  stones  carried  by  sea-weed  in,  188. 

MacCulloch,  Dr.,  termed  volcanic  rocks 
overlying,  21. 

,  on  consolidation  of  strata,  52. 

,  on  denudation,  81. 

,  on  compact  felspar,  92. 

,  on  trap  rocks  and  dikes,  104.  117. 

,  on  columnar  basalt,  111. 

,  on  passage  of  granite  into  trap,  123. 

,  on  granite  veins,  124.  126. 

,  on  altered  rocks,  109.  149.  288. 

,-on  isle  of  Arran,  294, 

Madeira,  dikes  in,  103. 

Maestricht  beds  and  their  fossils,  189. 

Magnesian  limestone  and  fossils,  240. 
242. 

,  composition  of,  28. 

,  concretionary  structure  in,  53. 

Malvern  hills,  rocks  altered  by  granite 
in,  288. 

Mammalia,  extinct,  found  with  living 
shells,  172,  173. 

Mammat,  Mr.,  on  faults  and  denuda- 
tion in  Ashby  coal-field,  82. 

Mammoth,  fossil,  172,  173. 


Mantell,  Mr.,  on  fossils  of  the  Chalk, 

188.  190. 

,  on  the  Iguanodon,  203. 

,  on  Portland  dirt- bed,  206. 

,  on  plants  of  the  Wealden,  209. 

Map  of  chalk  in  south  of  France,  196. 
Marble  described,  27.  134. 
Margarate,  term  explained,  142. 
Marine  formations,  how  distinguished 

from  freshwater,  42. 
Markerud,  strike  of  beds  not  altered 

by  intrusion  of  granite  at,  128. 
Marl  and  marl-slate  described,  28. 
Mechanical  deposits,  50. 
Megalichthys  Hibberti,  246. 
Meissen,  granite  covering  chalk,  near, 

291. 

Melanopsis  buccinoidea,  45. 
Melaphyre,  101. 

Menai  Straits,  tertiary  strata  near,  173. 
Mesotype,  102. 

Messenia,  puddingstone  of,  199. 
Metalliferous  veins,  128. 
Metamorphic,  term  whence  derived,  22. 

rocks,  general  character  of,  131. 

,  principal   members   of  this  class 

described,  132. 

,  their  origin,  21.  135. 143. 147, 148. 

,  stratificalion    of,     distinct    from 

cleavage,  137. 
,  kind  of  strata  from  which  some 

may  have  been  derived,  148. 
,  on  the  different  ages  of,  and  how 

determined,  293. 

,  of  the  Apennines,  Alps,  &c.,  295. 

,  must  be  old  before  they  reach  the 

surface,  299. 

,  order  of  succession  in,  299. 

,  why  less  calcareous  than  the  fos- 

siliferous,  301. 

Meyringen,  section  near,  298. 
Mica,  decomposition  of,  affords  silex  in 

solution,  60. 

,  composition  of,  102. 

Micaceous  sandstone  described,  26. 

Mica-schist  described,  133.  135. 

Microconchus,  245. 

Miller,  Mr.,  cited,  242. 

Millstone  grit,  244. 

Mineral  character  as  a  test  of  age  of 

rocks,  160.  274.  282.  294. 
Minerals  in  volcanic  rocks,  analysis  of, 

102. 

Mineralization  of  organic  remains,  55. 
Mingan  islands,  worn  limestone  pillars 

in?  194. 
Miocene,  term  whence  derived,  166. 

deposits  not  found  in  England,  173. 

volcanic  rocks.  276. 

Mississippi  R.,  deposits  in  estuary  of,  16. 
,  lagoons  alternately  fresh  and  salt 

at  mouth  of,  49. 

,  drift  wood  of,  260. 

,  delta  of,  261. 

Mitra  scabra,  179. 

Mitscherlich,  Professor,  on  augite,  93. 


312 


INDEX. 


Mitscherlich,  his  theory  of  Isomorphism, 
94. 

Moel  Try  fane,  tertiary  strata  011,  173. 

Monkey,  fossil,  180. 

Monomyary  mollusca,  44. 

Mons,  zigzag  flexures  of  coal  near,  68. 

,  unconibrmable  strata  near,  74. 

Morea,  cretaceous  rocks  of  the,  199. 

,  volcanic  rocks  of,  277,  278. 

Mosasaurus,  189. 

Mould,  formation  of,  82. 

Mountain  limestone  and  fossils,  243. 253. 

Mount  Battock,  granite  dikes  in,  125. 

Munster,  Count,  on  Solenhofen  fossils, 
221. 

,  on  fossils  of  the  Keuper,  &c.,  237. 

Murchison,  Mr.,  on  joints  and  slaty 
cleavage,  138.  141. 

,  on  tertiary  strata,  173. 

,  on  Brora  coal-field,  223. 

,  on  New  Red  sandstone,  236. 240. 

,  on  fossils  of  the  coal,  245. 

,  on  Old  Red  sandstone,  262. 

,  on  the  Silurian  strata,  265, 266, 267. 

,  on  Silurian  and  Cambrian  trap- 
rocks,  280. 

,  on  granite  of  Dartmoor  and  Mal- 

vern  hills,  288. 

,  on  granite  of  Brora,  291. 

,  on  geology  of  Arran,  294. 

Murex  alveolatus,  175. 

Muschelkalk,  fossils  of,  236. 

Naesodden,  greenstone  dike  of,  106. 

Naples,  tertiary  strata  of,  170. 

Nassa  granulata,  175. 

Nautilus,  figures  of,  179.  225. 

Keeker,  Mr.  L.  A.,  terms  granites  un- 
derlying igneous  rocks,  21.  148. 

,  on  metalliferous  veins,  128. 

,  on  the  Valorsine,  128,  129. 

,  on    metamorphic    rocks    of  the 

Alps,  298. 

Needles  of  chalk,  194,  195. 

Nelson,  Lieut.,  on  chalk  formed  by  de- 
f   composition  of  corallines,  186. 
I  Neptunian    theory  of  the    origin    of 
V-jocks,  153. 

Nefincea,  figures  of,  218. 

Nerinaean  limestone,  219. 

Nerita  granulosa,  45. 

Neritina  globulus,  45. 

Newcastle  coal-field,  great  faults  in, 
78.  82. 

New  Red  sandstone  group  of  rocks,  235. 

,  position  and  subdivisions  of,  236. 

,  origin  of  the,  242. 

New  Zealand,  212.  256. 

Niesen,  slates  of  the,  141. 

Niger,  R.,  delta  of,  209,  210. 

Nile,  R,  stratified  deposits  formed  by,  16. 

,  lagoons  at  mouth  of,  49. 

Ninety-fathom  dike,  78. 

North  cliffi  tertiary  strata  at,  173. 

Norway,  tertiary  strata  of,  171. 

,  granitic  rocks  of,  128,  129.  143. 


Norway,  Silurian  strata  in,  267. 
Nummulite  limestone,  198. 
Nummuliles,  figures  of,  198. 
Nyo'e,  new  island  destroyed  by  sea, 
116. 

Obsidian,  101. 

Oeynhausen,  M.,  on  granite  veins,  127. 

,  on  isle  of  Arran,  294. 

Old  Red  sandstone,  its  subdivisions  and 

fossils,  262. 

,  trap  rocks  o'f,  279. 

,  pi u tonic  rocks  of  the,  288. 

Olivine,  102. 

Oolite  formation, name  whence  derived, 

27.  213. 

,  extent  and  subdivisions  of,  213. 

,  fossils,  214  to  222. 

,  changes  in  organic  life  during  its 

accumulation,  217. 

,  signs  of  land  during,  221. 

,  volcanic  rocks  of  the,  278. 

,  plutonic  rocks  of  the,  287. 

,  metamorphic,  in  Apennines  and 

Alps,  296. 

,  and  Lias,  origin  of  the,  232. 

,  valleys  and  escarpments  formed 

by,  234. 

Ophiolites,  101.  277.  279. 
Ophites,  101.  278. 
Oppenheim  limestone,  43. 
Orbicula  reflexa,  220. 
Orford,  crag  strata  near,  174. 
Organic  remains,  age  of  strata  proved 

by,  160.  273. 

.     See  Fossils. 

Orthoceras,  figures  of,  254. 
Orthocerata,  on  structure  of,  268. 
Ostrea,  44.  183,  184.  218.  220. 
Outcrop  of  strata  explained,  72. 
Overlying,  term    applied  to  volcanic 

rocks,  21.  129. 
Owen,  Mr.,  on  fossil  bone  from  Enstone, 

222. 
Oxford  clay,  232,  233,  234. 

Palceoniscus,  241. 

Palaeontology,  term  explained,  164. 

Palaeotherium,  180. 

Paludina,  45. 

Pampas  have  been  raised  slowly,  63. 

Parasitic  fossils,  36. 

Pareto,  M.,  on  altered  Oolite,  295. 

Passy,  M.,on  chalk  cliffs  of  Normandy, 
195. 

Patagonia,  plains  of,  upheaved  gradu- 
ally, 63. 

Pearlstone,  101. 

Pebbles  in  chalk,  187. 

Pecopleris  Lonchitica,  247. 

Pecten,  figures  of,  192.  246. 

Pegmatite  described,  122. 

Peperino  described,  98. 

Pepys,  Mr.,  cited,  58. 

Petrifaction  of  fossils,  55. 

Petrosilex,  101. 


INDEX. 


313 


Peyrehorade,  numraulite  limestone  of, 
198. 

Pkasianella  and  cast  of  same,  55. 

Phillips,  Prof.,  on  grooved  surfaces  of 
faults,  78. 

,  on  joints  in  rocks,  141. 

,  on  the  Coal  strata,  243,  244.  246. 

,  on  the  Mountain  limestone,  253. 

,  on  Cambrian  fossils,  270. 

Phillips,  W.,  on  composition  of  clays,  26. 

,  on  faults,  78. 

Pholadomya  fidicula,  219. 

Phonolite,  101. 

Phyllade,  135. 

Physa,  figures  of,  45. 

Pid'dington,  Mr.,  his  analysis  of  the 
water  in  the  mud  of  the  Hoogly 
river,  58. 

Pmgel,  Dr.,  on  subsidence  in  Green- 
land, 63. 

Pitchstone,  101. 

Plagiosloma,  figures  of,  183. 

Planorbis  euomphalus,  45. 

Plants,  fossil,  of  the  Coal,  247. 

Plas-Newydd,  changes  caused  by  a 
dike  near,  107. 

Playfair,  on  rise  of  land  in  Sweden,  63. 

,  his  description  of  faults,  76. 

,  on  Huttonian  theory,  154. 

Plesiosaurus,  228. 

Pliocene,  term,  whence  derived,  166. 

,  period,  newer,  strata  of,  near  Na- 
ples, 170. 

,  in  Norway,  171. 

• ,  in  S.  America,  171. 

,  in  W.  Indies,  171. 

,  in  valley  of  Rhine,  172. 

,  in  Great  Britain  and  Ireland,  172. 

,  (older)  in  England,  173. 

,  volcanic  rocks  of,  275. 

,  plutonic  rocks  of,  why  invisible, 

283 

Plutonic  action,  145.  148. 

Plutonic  rocks,  described,  20.  118. 

,  their  relation  to  the  volcanic,  20. 

,  name  whence  derived,  20. 

,  age  of,  how  determined,  281. 

,  Recent  and  Pliocene,  why  invisi- 
ble, 283. 

,  of  different  periods,  283. 

,  relative  age  and  position  of.  284. 

.     See  also  Hypogene  rocks. 

Poikilitic  group,  236. 

Polished  surfaces  of  fissures  and  faults, 
75.  78. 

Ponza  islands,  globiform  pitchstone  in, 
113. 

Porcelain  clay,  26. 

Porphyritic  granite,  121. 

Porphyry  described,  96.  101. 

Portland  dirt-bed,  205,  206. 

Posidonomya  minuta,  238. 

Pozzolana,  composition  of,  53. 

Predazzo,  Oolite  altered  at,  288. 

Pressure,  consolidating  effects  of,  54. 

B  b 


Preston,  tertiary  strata  at,  173. 

Prestwich,  Mr.,  on  the  Coal  strata,  245 

Primary  fossiliferous  strata,  264. 

— — ,  horizontal  in  Sweden,  269. 

Primary  limestone,  134. 

Primitive  or  Primary,  term,  why  erro- 
neous, 23,  24.  154.  157. 

Producta,  figures  of,  241.  253. 

Protogine  described,  122.  133.  135. 

Puddingstone  described,  26. 

,  of  Hertfordshire,  52. 

,  of  Messenia,  199. 

Pumice  described,  97.  101. 

Pupa  muscorum,  46. 

Purbeck  beds,  200. 

Puzzuoli,  elevated  marine  strata  at,  170. 

Pyrenees,  bent  strata  in,  72.  74. 

,  lamination  of  clay-slate  in,  137. 

,  rocks  altered  by  granite  in,  145. 

286. 

,  chalk  of,  195.  197. 

,  trap  rocks  of,  278. 

Pyroxenic-porphyry,  101. 

Quadersandstein,  225. 
Q'uadrumana,  fossil,  180. 
Quaquu-versal  dip  explained,  72. 
Quartz  rock  or  Quartzite  described,  133. 
Quartz  veins,  129. 
Quiriquina,  island  of,  rocks  shivered  by 

earthquakes  in,  83. 
Quorra,  R.,  delta  of,  209. 

Radnorshire,  trap  rocks  of,  280. 

Ramsholt,  section  at,  175. 

Rancie,  altered  Lias  at,  145. 

Rathlin,  dikes  at,  108. 

Recent  period,  how  separable  from  ter- 
tiary, 169. 

,  formations  of,  in  different  coun- 
tries, 170.  172. 

,  volcanic  rocks  of,  275. 

,  plutonic  rocks  of,  why  invisible, 

283 

Red  sandstone  and  marl,  origin  of,  242. 

,  of  different  ages,  242. 

.  See  also  Old,  and  New  Red 

sandstone. 

Reptile,  living  marine,  of  the  Galapa- 
gos, 229. 

Reptiles,  fossil,  of  the  Wealden,  202. 
210. 

,  of  the  Lias,  227. 

,  of  the  Muschelkalk,  238. 

,  of  the  Magnesian  Limestone,  242. 

Rhine,  R.,  land  shells  drifted  by,  43. 

,  valley  of,  tertiary  strata  in,  172. 

Ribboned  jasper,  144. 

Riley,  Dr.,  on  fossil  reptiles,  242. 

Ripple  mark,  how  formed  (see  Fig.),  33. 

Rock,  term  defined,  14. 

Rocks,  all  divisible  into  four  contempo- 
raneous classes,  15.  22. 

,  aqueous,  described,  15.  25. 

,  volcanic,  described,  18.  89. 


314 


INDEX. 


Rocks,  platonic,  described,  118. 

,  metamorphic,  described,  131. 

,  how  to  detect  alumine  or  lime  in, 

26,  27. 

,  hardened  by  exposure  to  air,  52. 

,  their    particles    re-arranged    by 

chemical  action,  53. 

,  transported  by  ice,  86. 

,  how  to  be  studied,  94. 

,  altered  by  dikes  and  granite,  104. 

107.  123.  143.  145.  286.  294. 
,  different  ages  of  the  four  great 

classes  of,  151.  159.  271.  281.  294. 

,  classification  of,  15.  24.  157.  163. 

Rose,  G.,  on  hornblende  and  augite,  93. 
,  on  composition  of  volcanic  rocks, 

99,  100,  101. 

Ross-shire,  denudation  in,  81. 
Rostellaria  macroptera,  179. 
Rother,  R.,  buried  ship  in  old  channel 

of,  172. 

Rothliegendes,  240. 
Roththal,  section  in  the,  298. 
Rouen,  chalk  needles  near,  194. 
Rubble  explained,  83. 

St.  Abb's  Head,  curved  strata  near,  66. 

St.  Etienne,  vertical  trees  in  Coal  stra- 
ta at,  258. 

St.  Helena,  basaltic  dike  in,  111. 

Salbands,  105. 

Salisbury  Craig,  altered  strata  in,  109. 

San  Caterina,  bent  strata  near,  73. 

Sandstone  described,  25. 

San  Lorenzo,  isle  of,  Recent  strata  in, 
171. 

Sattel,  section  on  the,  298. 

Sauroid  fish  of  the  Coal,  246. 

Saussure  on  vertical  conglomerates,  64. 

Savi,  M.,  on  metamorphic  rocks,  295. 

Saxony,  aqueous  strata  rendered  colum- 
nar by  basalt  in,  140. 

Scania,  sinking  of  land  in,  63. 

Schorl,  analysis  of,  102. 

Schorl  rock  described,  122. 

Scoresby,  on  rents  in  icebergs,  140. 

Scoriae  described,  97.  101. 

Scrope,  Mr.,  on  Auvergne  volcanos,  91. 

,  on  volcanic  rocks,  98.  113.  275. 

Sea,  proofs  that  it  has  not  sunk,  but 
that  the  land  has  been  raised,  61. 

Sea-urchins,  figures  of,  37. 

Seale,  Mr.,  on  dikes  in  St.  Helena,  111. 

Sedgwick,  Professor,  on  garnets  in  al- 
tered rocks,  107. 

,  on  changes  caused  by  trap  dike, 

108. 

,  on  intrusion  of  trap,  110. 

,  on  granite  veins,  126. 

,  on  stratification,  joints,  and  cleav- 
age, 137,  138.  142. 

,  on  lower  New  Red  sandstone,  239. 

,  on  the  Magnesian  limestone,  53. 

241. 

,  on  the  Coal,  244. 

,  on  Cambrian  system,  264. 270. 281. 


Sedgwick,   Professor,    on    granite    of 
Dartmoor,  288. 

,  on  granite  of  Brora;  291. 

,  on  geology  of  Arran,  294. 

Segregation,  veins  formed  by,  129. 

Seine,  chalk  needles  in  valley  of,  194. 

Semi-opal,  infusoria  fossil  in,  40. 

Serpentine,  101,  102.  135. 

SerpulcB  on  fossils,  prove  slow  deposi- 
tion of  strata,  36. 

Sewalik  hills,  fossils  of  the,  180. 

Shale  defined,  26. 

Shells,  marine,  rules  for  recognizing,  44. 

,  freshwater  (see  Jigures),  44,  45,  46. 

,  common  to  rivers  and  the  sea,  46. 

,  amphibious,  46. 

,  terrestrial  (see  Jigures),  46. 

,  inferences  drawn  from  the  shape 

of  the  mouths  of,  46,  47. 

Sheppey,  isle  of,  fossils,  180. 

Shetland,  granites  of,  126. 

Ships,  fossil,  172. 

Sicily,  tertiary  strata  of,  62. 

Sidlaw  hills,  section  of  the,  65. 

,  trap  rocks  of,  279. 

SigillaricB,  figures  of,  248. 

,  erect  position  of  in  Coal  strata,  259. 

Silex,  in  solution,  source  of,  59. 

Siliceous  limestone  described,  27. 

Silurian  strata,  origin  of  name,  264. 

,  table  of  succession  of,  265. 

,  Upper,  fossils  of,  265. 

,  Lower,  fossils  of;  267. 

,  Lower,  trap  rocks  of,  280. 

,  in  Norway  and  Sweden,  62.  267 

269. 

,  horizontal,  269. 

,  in  N.  America,  268. 

,  granite  altering,  288. 

Skaptar  Jokul,  eruption  of,  274. 

Sky,  trap  dikes  in,  104,  105. 

,  columnar  basalt  of,  111. 

,  rocks  altered  by  trap  in,  109. 

Slaty  cleavage,  137.  141. 

Slickensides,  75.  78. 

Snowdon,  fossils  of,  270. 

Sodertelje,  recent  strata  at,  170. 

Solenhofen  fossils,  221. 

Solfatara,  rocks  of,  decompose,  147. 

Sorgenfri,  dike  at,  106. 

Sorting  power  of  water,  30. 

Spatangus,  figures  of,  37. 

Sphceruliles  agariciformis,  197. 

Sphenopteris,  figures  of,  204.  248. 

Spirt/era,  figures  of,  241.  253. 

Sponge  in  flint,  185. 

Spongilla  in  tripoli,  40. 

Springs,  calcareous,  50.  302. 

Staffa,  rock  of,  volcanic,  19. 

Stammerham,  cracks  in  clay  at,  208. 

Stations  of  species,  217. 

Steatite,  102. 

Steeple  Ashton,  fossil  coral  from,  215. 

Sternberg  on  fossil  coal  plants,  247. 

StigmarifE,  figures  of,  252. 

Stirling  Castle,  rock  of  altered,  109. 


INDEX. 


315 


Stokes,  Mr.,  on  lapidification  of  fossil 

wood,  60. 

,  on  structure  of  Orthocerata,  268.  i 

Stone-lilies,  fossil,  215,  21G. 
Stonesfield  slate,  fossils  of,  222. 
Strata,  term  defined,  15. 

,  original  horizontally  of,  30.  51.     j 

,  thinning  out  of,  explained,  31. 

,  parallelism  of,  30. 

,  ripple-marked,  33. 

,  gradual  deposition  of,  indicated 

by  fossils,  35.  42.  216. 

,  their  mineral  composition,  25. 

,  consolidation  of,  50. 

,  horizontal  at  great  heignts,  62. 

,  sometimes  reversed,  73.  159. 

age  of,  how  determined,  159.  271.  I 

281.  294. 
,  fbssihferous,     chronological     ar-  ! 

rangement  of,  157.  163. 

,  oldest  sometimes  horizontal,  2G9. 

,  conversion   of   fossihferous  into 

metamorphic,  295. 

Strathaird,  fissures  caused  by  decom- 
posed trap  dikes,  104. 
Stratheden,  trap  rocks  of,  279. 
Slrathmore,  valley  of,  65.  2G2. 
Stratification,  forms  and  causes  of,  15. 

28.  32,  33.  35.  64.  74. 

,  proof  of  aqueous  origin,  16. 

,  of  deposits  in  lakes  and  estuaries, 

15,  16. 

,  planes  of,  how  far  parallel,  30. 

,  distinct  from  cleavage,  137. 

Stratum  defined,  15. 

Strickland,  Mr.,  on  tertiary  strata,  173. 

Strike  and  dip  explained,  69. 

,  sometimes  not  altered  by  intruded 

granite,  128. 
Stromboli,  273.  276.  283. 
Studer,  M.,  on  alternation  of  gneiss  and 

fbssiliferous  rocks  in  the  Alps,  298. 
Stutchbury,  Mr.,  on  fossil  reptiles,  242. 
Subapennmes,  interstratified  tuffof,  276. 
Subsidence  of  land,  63,  64. 

,  in  Cretaceous  period,  193. 

,  in  Wealden  period,  204.  212. 

,  in  Carboniferous  period,  257. 

Succinea  elongata,  45. 

Suffolk,  freshwater  strata  in,  173. 

,  Crag  of,  described,  173. 

Suishnish,  trap  rocks  of,  105. 
Superior,  Lake,  recent  deposits  in,  52. 
Superposition,   relative    age    of  strata 

shown  by,  159. 
Sutton,  section  at,  175. 
Sweden,  gradual  rising  of  land  in,  63. 

,  Recent  and  Tertiary  strata  of,  171. 

,  Silurian  strata,  horizontal  in,  62. 

267.  269. 

Swiss  Alps,  altered  rocks  of,  296. 
Syenite  described,  121. 
Syenitic  greenstone  described,  96.  101. 
Synclinal  line  described,  66.  72. 

Table  Mountain,  stratification  of,  62. 


Table  Mountain,  granite  veins  in,  125. 

Talc,  102. 

Talcose  gneiss,  135. 

Talcose  granite,  122. 

Talcose  schist,  135. 

Tattingstone,  crag  strata  at,  174. 

Tephrine,  101. 

Tercis,  chalk  of,  197. 

,  chalk  and  volcanic  tuff,  alternat- 
ing at,  277. 

Terebellum  fusiforme,  179. 

Terebrahdce,  fossil  figures  of,  183.  192. 
220.  266. 

Teredina,  fossil  wood  bored  by,  38. 

Teredo  navalis,  wood  bored  by,  38. 

Terminology,  199. 

Tertiary  formations,  their  relative  posi- 
tion, fossils,  &c.,  164. 

,  divisible  into  four  groups,  165. 

,  how  classified,  165. 

,  of  different  countries  described, 

169. 

,  how  distinguished  from  Recent, 

170. 

,  volcanic  rocks,  275. 

,  plutonic  rocks,  284. 

Testacea.     See  Shells. 

Thermal  ocean,  theory  of,  155. 

Thinning  out  of  formations,  162. 

Thirria,  M.,  on  the  Oolite,  233. 

Thun.lakeof,  land  shells  drifted  into,  43. 

Thurmann,  M.,  on  the  Swiss  Jura,  71. 
218. 

Tisbury,  fossil  coral  from,  214. 

Toadstone,  101. 

Tourmaline,  102. 

Tournedos,  chalk  needles  at,  194. 

Trachyte  described,  96.101. 

,  analogy  in  composition  of  granite 

and,  123. 

and  basalt,  relative  position  of,  274. 

Transition  strata  and  fossils,  152. 154.264. 

Trap  conglomerates,  116. 

Trap  dikes  described,  103. 

,  rocks  altered  by,  104,  107. 

,  their  abrupt  termination  caused 

by  denudation,  115.  273. 

Trap  rocks  described,  89. 

,  name  whence  derived,  89. 

,  step-like  appearance  of,  90. 

,  changes  caused  by,  104.  107. 

,  intrusion  of,  between  strata,  110. 

,  their  relation  to  modern  lavas,  114. 

117. 

,  pass  into  granite,  123. 

,  regarded  by  Werner  as  aqueous 

deposits,  153. 

,  on  the  different  ages  of,  271. 

.     See  also  Volcanic  rocks. 

Trap  tuff  described,  98. 

Trass,  101. 

Travertin  deposited  by  springs,  50. 

Tree-ferns,  figures  of,  249. 

Trees  in  Coal  strata,  erect  position  of,  257. 

Treuil  coal-mine,  vertical  trees  in,  258. 
Trigonia  gibbosa,  218. 


318 


INDEX. 


Tripoli  composed  of  infusoria,  39. 
Trochus,  and  cast  of  the  same,  55. 
Tronstad  Strand,  section  on  beach  at,  129. 
Trough,  or  basin,  described,  66. 
Tuff,  volcanic,  19.  98. 

,  imbedding  of  fossils  in,  273. 

Tufaceous  conglomerates,  98. 
Turner,  Dr.,  on  combinations  formed  by 

mineral  matter  when  in  a  nascent 

state,  58. 

,  on  source  of  silex  in  solution,  59. 

Turrilites  costatus,  184. 
Tuscany,  volcanic  rocks  of,  276. 

,  calcareous  springs  of,  302. 

Tynedale  fault,  78. 

Unconformable  stratification,  74. 

Underlying  rocks,  term  proposed  for 
granites,  21.  129. 

Unio  litloralis,  44. 

Upheaval  of  extensive  masses  of  hori- 
zontal strata,  62. 

Upper  New  Red  sandstone  and  fossils, 

Val  di  Noto,  volcanic  rocks  of  the,  115. 

Valley  of  Bray,  182.  211. 

Valleys  of  denudation,  80. 

Valorsine  granite,  veins  of  the,  127.  298. 

Valvata,  45. 

Veins,  granitic,  193. 

,  metalliferous,  128. 

,  of  segregation,  129. 

.     See  Dikes. 

Velay,  volcanic  rocks  of,  18. 

Vertical  strata  parts  of  great  curves,  64. 

Vesuvius,  273.  276. 

Vicentin,  columnar  basalt  in  the,  112. 

Virlet,  M.,  on  corrosion  of  rocks  by 

gases,  147. 
,  on  cretaceous  rocks  of  the  Morea, 

199. 
,  on  volcanic  rocks  of  the  Morea, 

277,  278. 

Vivarais,  volcanic  rocks  of,  18. 
Volcanic  eruptions,  number  of  in  a 

century,  283. 
Volcanic  dikes  described,  103. 

,  rocks  altered  by,  107. 

Volcanic  grits,  98. 

Volcanic  rocks  described,  18.  89.  92. 99. 

produce  a  fertile  soil,  90. 

,  analysis  of  minerals  found  in,  102. 

,  their  fusibility,  99. 

,  their  relation  to  trap,  114.  117. 

,  denudation  of,  shown  by  dikes,  1 15. 

,  submarine,  19.  115. 

,  on  the  different  ages  of,  19.  271. 

,  age  of,  how  determined,  271. 

,    See  also  Trap  rocks. 


Volcanic  tuff  described,  19.  99. 

,  imbedding  of  fossils  in,  273,  274. 

Volcanos,  extinct,  of  different  coun- 
tries, 18,  19.  91. 

,  cones  and  craters  of,  how  formed, 

91. 

,  all  near  the  sea,  116. 

Vollzia  brevifolia,  237. 

Volutes,  figures  of;  177.  179. 

Von  Buch,  on  rise  of  land  in  Sweden, 
63. 

,  on  granite  of  Norway,  129.  289. 

,  on  altered  Oolite,  288. 

Von  Dechen,  M.,  on  granite  veins,  127. 

,  on  isle  of  Arran,  294. 

Wacke  described,  101. 

Waller  cited,  155. 

Water,  sorting  power  of,  30. 

,  levelling  power  of,  30,  31. 

Watt,  G.,  experiments  of,  146. 

Wealden  strata,  position  and  subdivi- 
sions of,  200. 

,  fossils  of,  201  to  204.  209,  210, 

211. 

,  passage  of  beneath  chalk,  203. 

,  how  formed,  204.  208. 

.extent  of,  209. 

,  age  of,  211. 

Webster,  Mr.,  on  dirt-bed  in  Portland, 
206. 

Weinbuhla,  granite  of,  covering  chalk, 
291. 

Wenlock  formation,  267. 

Werner,  his  classification  of  rocks,  152. 

West  Indies,  recent  and  tertiary  strata 
in,  171. 

Westphalia,  cretaceous  rocks  of,  193. 

Whin-Sill,  intruded  trap,  110. 

Whinstone,  101. 

WThite  Mountains,  granite  veins  in,  283. 

Whitestone,  135. 

Witham,  Mr.,  on  fossil  coal  plants,  56. 
260. 

Wood,  recent  and  fossil,  drilled  by  per- 
forating mollusca,  38. 

,  fossil,  magnified  portion  of,  56. 

,  experiments  to  illustrate  the  petri- 
faction of,  57. 

,  rate  of  its  lapidification,  60. 

Wrekin,  trap  tuffs  of  the,  280. 

Yarmouth  sands,  new  channel  in,  178. 
Yorkshire,  Oolite  of,  223. 

Zamia,  fossil,  in  Portland,  205. 

,  recent,  figure  of,  205. 

Zechstein  and  fossils  of,  240,  241. 
Zoophytes,  limestone  formed  by,  50. 
,  fossil,  in  Oolite,  214.  216. 


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